The Plus/4 only needs custom controls for starting/stopping the motor... (power too of course)
If you wired this to "always on", and left enough space (or hit a key quick enough) then you could hook in a normal tape/cd deck.
It would be quite neat, protect those tapes...
OR..... perahps a .TAP to real tape convertor, then store the ones you have on here in a more compressed form. This is a better "backup" since you can reproduce the tape if you need to.... even to CD I guess...