Why use a boot floppy. I hear your quite a skilled hardware builder. Then it should be no problem for you to make your own ROM that has the drivers gor your IDE drive. I thought the A500 has 256k Rom and mirrors its ROM in the 256k after the rom space. So it should be possible to create somthing that takes advantage of that.
Or an A600 3.1 Rom. I hear this work on the A500 too. You might be able to replace SCSI.device from that one.
Anyway, good work and congratulations.