Thanks. The guys over at
http://www.necromancergames.com have been giving the rules a thorough thrashing, and I think I'm approaching the final 1.1 version.
The latest (today's) version is at
http://members.aol.com/geocorona/zothique-d20-v1.pdf
DM's will have to be familiar enough with Thasaidon to know he doesn't give away power for free, and think of their own crafty way of receiving payback.
Perhaps an answer to the arcane vs. divine spell issue would be to have certain arcane spells ALWAYS granted by the grace of a certain god, with no regard for the alignment of the caster, but always with a cost. This would make all spellcasters at least somewhat clerics, and help fuzz the border between the two as CAS did.