If you do have the original ISA adapter board lying around somewhere you could certainly use it in one of those oddball old NuBus cards that have ISA slots... using it on anything newer that doesn't have the slot would be more of a problem. In principle most PC compatibility cards for a Mac probably have the signals for an ISA bus present on them somewhere, but that somewhere might be entirely inside a proprietary Southbridge IC. In any case, having it connected that way wouldn't make it accessible to the Mac, of course, unless you used some sort of client/server software setup.
In any case, what do you want this for? If it's to read PCMCIA flash cards on a pre-PCI Mac then the SCSI widget is probably your only practical choice. Or are you looking to host a PCMCIA peripheral, like a network card? If it's that, well, the SCSI widget won't help you either. I'm *pretty sure* that none of them are technically capable of supporting anything other than Linear Flash or (more usefully) ATA disk-emulating devices. (Which would include most flash cards over a few MB in size.)
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