Hi Anthon,
There is a demand for ADB devices, mostly from us. It may not be big, but a demand it is. Make a lot and see how it goes. You probably won't make any money on the first attempt, but you will gain experience.
As you learn more, the possibility of building other ADB devices becomes more feasible. I reckon, for example, that an ADB KVM would sell well. If you could put 12 ports on it, I'd say many would be prepared to pay $100 for it.
I, myself, would love some devices that cross the ports-divide: for example, a SCSI-FW converter. If I could have a FW-SneakerNet drive that I could use on my SCSI Macs that'd great. SCSI on PCI Macs is not so urgent as I can use a PCI SCSI card. What I would really love would be an enclosure with both SCSI and FW on it.
I am excited by your current project,
aa