With the new MH2 Spoilers I got motivated to look through the discard/Madness cards, that currently exist:
White and Green are excluded, because while white has some good Discard outlets, there are no relevant white Madness cards and Green has basically just Arrogant Wurm and that's it. Apart from Wild Mongrel it also lacks discard outlets, so I will focus on the Grixis colors.
Before I start listing cards, some basic rules of thumb. Discard Outlets should in the best case be repeatable and cost as little mana as possible (0-1), to actually make Madness a tempo advantage. When you pay 2 mana for the discard effect to cast a Reckless Wurm with it, you are still paying 5, so it's no real advantage.
(Staple = No big problem casting this without Madness and/or absolutly bonkers with Discount // Cubeable = Hard to cast/Garbage without Discarding it or not that great when cast with Madness)
Circular Logic: +Ok to hardcast if necessary. A Counterspell for U is extremely broken for obvious reasons.
Obsessive Search: +Ok to hardcast if necessary. A very low opportunity cost. It basically just gives you a "free" activation of any discard abilities.
Psychotic Episode: +Ok to hardcast if necessary. A pretty decent discard spell, that gives you a lot of information, even the top card, and can be cast during your opponents drawstep with instant speed discard and even hit lands.
Revolutionist: +Ok to hardcast if necessary. For 4 mana it's a very good deal.
(Staple = Repeatable, cheap to activate and getting something in return // Cubeable = Non-repeatable, or simply there to discard/you don't really get anything for discarding. Also overpriced Discard effects)
Aeromoeba: The discard effect doesn't do much, but it's free and repeatable to activate.
Thalakos Scout: It's technically "free" to discard, but it always cost 3 beforehand.
Dreamscape Artist: Not a bad effect to get a Harrow with every discard, but it making every Madness card cost 3 more isn't all that great.
Aquamoeba: Cheap and Repeatable, but you basically get nothing in return.
Tortured Existence: This even allows you return killed Madness creatures and discard them again for additional value.
Undertaker: The difference to Tortured Existence is, that you can discard lands, when you run out of madness cards and turn them into creatures. The downside is a fragile 1/1 body with summoning sickness and T as a requirement.
Plague Witch: Cheap, Repeatable, but sadly -1/-1 doesn't compare to drawing a card or returning a creature from your GY to your hand.
Putrid Imp: If you are really desperate for discard outlets it doesn't get any cheaper than this, but the card itself and the value from discarding are quite lackluster.
Crypt Lurker/Rank Officer: Seems like a great deal in theory, but having to pay the madness cost in the same turn, where you already spent 4 mana makes these extremely slow.
Rotting Rats: Pseudo Repeatable with Unearth. The nice thing is, that your opponent also has to discard, while you get to discard, but requireing 2 mana beforehand is a little bummer.
Trespasser il-Vec: Cheap and Repeatable, but without constantly discard to it, the 3/1 body won't really cut the deal.
Olivia's Dragoon: Cheap and Repeatable and with an initial cc of 2 quite affordable, sadly getting flying every now and then isn't that amazing.
Vampire Hounds: It definitely is cheap and repeatable, but it requires creatures and the base stats of 2/2 for 3 aren't amazing.
Bone Shards: Not repeatable, but only paying B is definitely a nice deal for killing a creature of your choice in return.
Sparkspitter: The only red card that checks all the boxes. Sadly the initial cmc 3 hurt the curve a little, but getting a "free" Spark Elemental with every discard should make up for the lost tempo quite fast.
Faithless Looting: Pseudo Repeatable once with Flashback, but the important part is, that you get your first discard for just R.
Gathan Raiders: Not repeatable, but it doesn't get much better than 0 mana at any time you like, while you also get a decent body.
Insolent Neonate: Not repeatable, but a onedrop you can drop nice an early and push through some points of damage for free, before saccing it.
Lightning Axe: Sadly absolutely horrible without discarding a card, but the deal you get when you do feels quite nice.
Mad Prophet: Repetable, but only cheap in activation cost. 4 mana is rather expensive, before you can even start to discard your first card.
I think Vampire Hounds and also Erstwhile Trooper are too much of a one trick pony for me. How many Madness creatures do you need in your deck, to activate this ability for value more than once? Your opponent can always put a 2/2 in front of these and they will always trade for a creature, either on board or in hand. I think they would be broken if they could discard any card, but with the creature-only clause they are too weak, IMO.
Additionally, I'm sure certain mechanics "play nice" with the Madness archetype as well - providing pseudo-support, if you will. Stuff like Retrace, Flashback, Jumpstart, Cycling, etc... can either provide stuff you wouldn't feel horrible about discarding or stuff that provides easy discard outlets on the side.
Is your Madness experiment still going, and have you incorporated Terminal Agony? Seems like an auto-include, but I didn't see it in your initial list of cards despite other MH2 stuff being listed.
I don't get to draft all that often. I think we had about 4 drafts since the inclusion and so far it has been fine, but I'm still a bit ambivalent. We don't always draft with 8, but rather 4-6 with 50%-75% of the cube. In several drafts multiple people tried to draft the archetype and even without 100% of the cube it could get a little thin, even though I tried to go rather deep into the archetype. Another thing is, that cards like Merfolk Looter can be picked up by everyone and don't necessarily end up at the madness drafter.
Not running Terminal Agony has to do with my cube design philosophy. I don't want to dictate people what they do and I feel like, when I put this into the multicolor slot drafters would think, that they had to draft madness in BR, but that's not what I want. On the one hand there are non-madness BR decks, which work just as well and in addition to that UR and UB are about evenly supported, without a multicolor card. In addition to that Madness cards have the big problem that most of them are pretty useless for non madness decks and with having a multicolor card just for that archetype I feel like I would disadvantage non-Madness BR decks.
And last but not least one argument for the madness cards, that made the cut was that they should, if possible, be reasonable to play without madness and a 4 mana sorcery speed removal is pretty bad.