Not happening, imho. Sorry. I'd rather see an illithid only class instead.
Illithiad (Squids) open for Battlemage.
Now that sounds like a GREAT idea. Maybe some type of mental caster akin to psions but different.
I would see an illithid bmage as extremely powerful - with the 1 prac learn it helps with low bmage hp gain (illithids get decent base for mages after training). THe defenses of a bmage also help a LOT as they are not based on weapons (3/4 anyway..) Mostly though, their mana would make them hard to stop - a bmage relies on mana for upkeep of a lot of their major spells. You can only hunt for so long - which is why a long drawn out battle never favours a bmage. With so much mana (and such great gain) I think illithid bmages would be a little over the top.
THis is the issue for a lot of other classes on ilithid - the hp/mana just makes them VERY strong. We've already seen players ONLY get +hp gear and do well on their necro's - allowing that on bmage or invokers and I think we'd have a real issue balance-wise.
RP wise - well, nerco and psion fit. I don't know if a lot of ther classes do.... Nekky makes a valid point regarding illithid RP.
L-A
A slowed illithid gains 800 mana per hour 
I already suggested in an earlier thread that illithids should be able to be telepathy psions from creation, due to their mastery of psionic techniques.
If I remember correctly a lot of people thought it was a bad idea, but to me it makes perfect sense.
If I remember correctly a lot of people thought it was a bad idea' date=' but to me it makes perfect sense.[/quote']
I actually think a lot of people agreed with your idea