I'm pretty sure the ROMs loaded a boot sector booter and jumped into that, it's been a long time but I'm pretty sure that's what we handed over to Apple
It's possible that later A/UXs required booting MacOS - but when we started we got 1/2 the initial prototype production run of Mac 2s (no cases, bugs in pals, probably no standard ROMs yet)
Our standard agreement for porting Unix to new hardware (we did something like 150 different boxes, not just macs) required a minimal ROM monitor that could download code from a serial port (we could provide source), we'd take it from there, write S/A code to store data on the harddrive, download a file system, start downloading kernels into ram debugging the disk and serial drivers until we got a shell prompt ... then moving development onto the box