Actually, the PM engine is based on Everything 0.8.3. It would be good if it were 0.9-based - all the overlapping work done at this site, Everything2, and at EveryDevel could be shared back and forth. Alas..
The reasons for not opening this site's codebase have been discussed at length before, and while you disagree, I largely follow the arguments and agree with the conclusions on the side of the gods. This doesn't mean I'm satisfied with the status quo, and I can't imagine any of the gods is either. Noone is trying to spite you or wants to keep a lid on things; I'm pretty sure that at heart everyone involved understands and shares your sentiment of wanting everyone who feels the urge to be able to contribute. Unfortunately this just doesn't seem feasible right now.
The most promising endeavour in the long term would be to port PM to E core 0.9; ultimately, then, the work here would be shared back to EveryDevel and could be available publicly from there. Any contributions would flow back and forth, and whoever wanted to contributed could do so by improving the base E core. This would benefit far more people than just PM, as well.
Makeshifts last the longest.