I couldn't get Disk2FDI to work - but in fairness I wasn't totally clear on what the demo version was actually doing.

ADTPro is one of the coolest things I've ever seen, and that does work.  I haven't gone beyond sending the client because I don't have any spare disks, but for sure that seems to be the best practical solution since - at least in theory - it allows access to the drive.  I'm not sure what limitations it has in terms of getting sort of universal or portable data from the cassette port.  But their inadvertent choice to just go with a minijack port built-in, rather than Commodore's model for instance, has left these computers (II, II+, IIe) indefinitely serviceable - in theory.  I just think that's really cool.  

Sadly I tossed my IIgs and IIc years ago - although in ADTPro terms keeping the IIe may have been a smart decision.

In any event, thanks.