I've done some experimenting, and it looks like playing tapes back is way easier than recording them, but both can likely be done without too much trouble. Investigating my tape deck, I found that when recording the head was receiving a 32V p-p 106kHz sine wave. I had a 1kHz source hooked to the input and I wasn't seeing anything but pure wave so I'm not sure what was going on there but it could have been a channel mixup so I'll look into it further this weekend, and try to find more parameters to work with.
I was initially under the impression that the 2nd head in the unit is for recording, and the main one was playback but in fact that is an erase head, play/rec is all done on the center head. This is disappointing since the device can't be used as a delay without heavy modification. Looks like a looper will be the only function.
My homemade loop tape seems to be working so I'll hopefully be able to do some more tinkering soon. My tape deck is malfunctioning and won't even record on unmodified tapes, so I need to find another one so I can make some sweep signal test tapes for developing and tuning the playback EQ.