That's a nice idea in theory, but won't the people who like 3.5 the way it is, who want the APs to stay 3.5 compatible, still be playing their 3.5 game with their 3.5 rulebooks (or Arcana Evolved with their AE rulebooks or Iron Heroes with their IH rulebooks, etc.) during the playtest period - and thus, not contributing to the playtest? Why would I put in the time and effort to playtest the game and struggle to keep Pathfinder as close to D&D 3.5 as possible when I've already got D&D 3.5, or Arcana Evolved or Iron Heroes or True20, etc. and am perfectly happy with it and just want more APs that I can use with my game?
It just seems to me like you'll be self-selecting for people who WANT Pathfinder to diverge from the D&D 3.5 rules with the open playtest and that the results won't necessarily tell you anything about the people who just want the APs to stay as they are.