I did not try this with a Plus, but any of my Macs that was born with a SCSI interface boots happily from flash memory connected via an appropriate adapter. I tested several adapters with different flash memory cards in Macs like PB180, ColourClassic, P475, PB1400, and a platinum G3. Find some hints about this topic in related threads and in the 68kMLA Wiki. The page "Using flash memory" might provide a good entry.