It doesn't seem to be illegal to replace (temporary) one or more DLLs in XP for the reason of interoperability verification.
Distribution of such replacement DLLs to the public for the same reason (and with proper EULA) should not be a problem as well.
Of course, calling such activity "patching" the OS is just inapproprite and wrong in nature.
Regarding free software licensing in general, in my opinion end-user should accept 100% responsibility (not 99.999%) for downloading and using it, including verification of patent infringemency, copyright violations, complience to the local laws, etc.
I mean a free software EULA should guarantee 100% prevention of any legal action against software developer and publisher.
BTW, can anybody refer me to such license?
Unfortunately, GPL does not provide such protection.