Couldn't you just double the level/HD of the all the NPCs and Monsters in the adventure? Then add a Book of Vile Darkness styled Lolth (or Avatar of Lolth) into the finale?
I'm not sure. The issue is that I don't own it yet. I'm wondering if it's even worth the bother if it's going to be too much work to bend, fold, spindle and mutilate to the design I want.
It wouldn't be so bad of a true Solo, but for level 20 gestalts it really doesn't seem like a good challenge. Inserting tougher monsters and stuff is the easy part. But at that level you either have to place in ridiculous restrictions or come up with some genius way for the PCs to experience the module and not just skip to the end.