Ah, you can moan about PCs because there is much information about how they go about their business.
Where as the Mac it's all closed apart from the bits Apple thinks you need to know.
Nobody I know boots a 68k Mac by themselves, it's all done through Mac OS ROMs first, so A/UX, NetBSD, Linux all really use System 7 as a first stage boot loader.
That said, the SE is a fairly simple beast and it depends what hardware you want to drive, note that NetBSD etc all don't bother with floppy on 68k, it's SCSI, IDE or nothing.
So what are you *really* trying to do?