Re-reading, I found some excellent stuff on working Possessors-inside-the-Sorcerer (and also parasite demons) in Sorcerer’s Soul.
Like most everything in the Sorcerer line (or, really, everything by Ron Edwards in general), it made a vague sort of sense before but seemed quite obfuscated — then we did some playing — then I re-read it and it all seems terribly useful, insightful, and crystal clear.