Only way I can think of, is that you make a copy of briliance aura, make it affect only enemy units, and make mana regeneration bonus negative (using shit + click in object editor you can add negative values). Then create a spellbook which contains that new aura. In map initialization make a trigger that disables the spellbook for the player on start.
Then make an trigger:
Events
Unit - A unit Learns a skill
Conditions
(Learned Hero Skill) Equal to Brilliance Aura
Actions
Player - Enable SPELL BOOK for (Triggering player)
Unit - add SPELL BOOK for (Triggering Player)
Unit - Set level of CustomAura for (Triggering unit) to (Level of Brilliance Aura for (Triggering unit))
Player - Disable SPELL BOOK for (Triggering player)
I think this should do it, but have someone more expirienced than me check it.
EDIT: here I uploaded map with the ability. Like I said before, someone should re-check it if it's 100% legit