[Taqwus]

During the turns spent merely sieging, the defender can try to assassinate the besiegers (if he has any assassins... and star children are cheap) while the besieger cannot. A storm is a single battle, 'tho, and assassins are just like any other unit if they participate.
The defender's troops are also not known (would a Stone Sphere work? Probably. Don't otherwise go Astral Probing R'lyeh 'tho.), hence the suggestion of a suicide scout.
If you have Earth Attack available, that will work even underwater and during a siege. That's pretty high up 'tho. Mind Hunt would work, except that you'll probably end up with feebleminded mages because R'lyeh is astrally proficient.
Tritons are a good idea; if you don't have scads of mindless or high-MR troops, another counter for mind-bLasting is having numerous cheap, incredibly mobile troops that spend a minimum time getting paralyzed before they even reach the illithids.