Many rules use Will to solve "mental combat". For instance, combats vs. the magical guardian of Scrying guardian spells from the "Scrying Guard" list use will contests. Combats vs. spirits do as well. Based on this, one may want to use will contests for possession rather than a mere RR roll.
"Will Breaker" offers interesting options (only applicable to spells from the list itself but that easily be widened to all M spells) but a GM would probably have to extend will contest rules so that a character may lose will because of such a contest, and gain it back with time.
In my world, some creatures may initiate will contests.
* One "normal" round equals to five "phases" of will contest,
* At each phase, the losing character loses a number of will points equal to the difference between the will rolls,
Ex.: PC (will 156) rolls a 85 for a total of 241,
NPC (will 184) rolls a 48 for a total of 232.
--> NPC loses 9 will points and has his will drop to 175.
* If there are several will contests at the same time, all characters only roll once and lose points based on difference with each of his opponents,
* Will contests are tiring: a character loses 3 exhaustion points per round of contest will, and takes a 'A' stress critical on rounds 4-6, 'B' on rounds 7-9, 'C' on rounds 10-12, 'D' on rounds 13-15 and 'E' on each subsequent round,
* A character gains back 10 will points per round he's not having a will contest, except if it had reached 0 (or below), in which case he gains 10 will points per hour,
* As long as his will is below 0, a character's level is divided by two where making RRs vs. M spells.