I wonder if Valve has hackathons and if the Proton guys want a new challenge.
At any rate, we'd love to at least have your patchset in HaikuPorts, even if it's not quite ready for prime-time yet.
However if mmap is more significantly broken then Haiku needs work - Wine does need the ability to map things into the address space at the requested locations and actually get it.
mmap on a fixed address should work just fine, unless the address is outside the normal range or in an already mapped region, which seems to be a problem on 32-bit systems. I think there are some emulators we have ports of which already use MAP_FIXED quite a bit.