If you "mutate" Glowstone Recluse with Almighty Brushwagg, the resulting creature will have reach and Glowstone Recluse's "When this creature mutates..." ability, and the latter ability will trigger (C.R. 702.139d-e). This is the case regardless of whether you "mutate" Glowstone Recluse on top or on the bottom.
RULES OF MAGIC :
http://magic.wizards.com/en/game-info/gameplay/rules-and-formats/rules
The reach ability is immediate, since it is part of the Recluse's copiable values, but the counters have to wait for the trigger to resolve.
Former Rules Advisor
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(The Gamers: Dorkness Rising)
"Any sufficiently analyzed magic is indistinguishable from science."
(Girl Genius - Fairy Tale Theater Break - Cinderella, end of volume 8)