Building a Linux or Cygwin m68k-amigaos cross is pretty straightforward, so I can't imagine a Linux or Cygwin m68k-sysv4 cross being much different. And since a sysv4 cross shouldn't require patching the gcc sources. . . . Sounds like a project.
[color=FF0000]EDIT: The m68k*-sysv4 targets were removed quite some time ago, and from what I've read, AMIX doesn't properly support the m68k-elf application binary interface. D'oh![/color]
I've also been toying with adding MMU support to WinUAE. I'm using the uae-8.20 MMU patches as a starting point. The code compiles (good start), but I've still got logic errors to work out. WinUAE's CPU emulation has changed non-trivially since uae-8.20.0. And Toni should be releasing a new source snapshot soon, so there's that to contend with as well.
Trev