Ok so in resume we have the following PS/2 mouse bugs:
NeXTStep and OpenStep PS/2 driver receives no movement at all from the mouse (Darwin PS/2 driver seems to be made from scratch so no helpful here). MS-DOS Mouse driver hangs the whole machine. Windows Me Setup mouse driver receives no movement at all from the mouse. Old XFree86 PS/2 mouse driver receives no movement at all from standard protocols (PS/2, IMPS/2) or random movements (Microsoft). Hydrogen OS mouse driver does not work.
While the NeXTStep PS/2 bug is known from QEMU 0.7.0 (which used Bochs BIOS), the other ones are new (as far as I know) so they may be or may not be caused by SeaBIOS.