Maybe there could be some sort of model that's not fully closed or open.
For example, lets say I find a bug that's making life a misery to me (there seems to always be 1-3 of those per RTM). Then lets say I submitted this bug and MS decided it was not worth fixing because not enough people reported it or fixing it was high risk.
Now lets assume MS wanted to find some solution to this that allowed people to fix those bugs in some restricted manner (and of course you'd become responsible for maintaining the fix if future official update should cause incompatibility with your patches). If I was responsible for this solution I'd just tell people to send suggestions for how to handle this scenario and then filter them down until something was found. One such suggestion might be to have people who really want to fix their pet bug, pay some sum to cover fees of setting up NDA and then just hand them the code for a particular module which they need to fix the bug (or which they think they need - to avoid spending money on MS side to identify where the bug really is). And then have tool that can turn this "partially available source"-modification into some kind of patch that gets applied to the windows .dll just after it gets loaded (it would be signed in a way that only let it load on your computer).