I dont think your solution would work. If you loadlibrary an absolute path DLL so 2 dll are in the process with the same name, I think it is "undefined" which DLL will be used in a future DLL loading that is dependent on that base name. I dont know if Windows is smart enough to try the 1st DLL with that base name, if the imports can not be resolved, then try 2nd DLL with same base name.
Only SxS, but that isn't Win2000 compatible, and SxS generally isn't used in Mingw, or pure LoadLibrary/GetProcAddress would work. I am not saying that SxS is impossible with Mingw, just that I've never seen it done. ld doesn't have any manifest specific flags in --help.