It might be interesting to see how much of the Plus works with the old ROM. Obvious SCSI won't work, but... other than that addition the machines might be similar enough for the Plus to make it to the desktop if you boot it with an appropriate single-sided System disk. (It in particular might be interesting to see if the 64k ROM freaks out when presented with 4MB of RAM.) ;^)
Heh. If you're really bored you could rewrite this SCSI driver software to support the Plus's built-in SCSI, assuming the Plus boots with the 64K ROMs. The hardware is actually *very* similar. (It's one of the reason the Mac Plus' SCSI is so slow, even compared to the otherwise similar SE.) If you did that you'd make someone on this board SUPER HAPPY since then he could add a Dove SCSI board (which matches the Plus' memory map) to a 128k and have mass storage while preserving the sacred 64K ROMs for all eternity.