The memory layout of Haiku x86_64 is identical to that of Linux. Perhaps you should try there first?

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.

You could just bypass the preloader completely. It exists only to work around the Linux ASLR implementation which would otherwise tend to place DSOs randomly in the address space, potentially overlapping regions that Windows keeps reserved. If Haiku doesn't implement ASLR, or lets you easily disable it, or alternatively would accept patches to ensure its address space layout is always Win32 compatible (better!) then the preloader could be disabled.

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.