Well I tried using a utility called CAQ2WAV which was made for converting Mattel Aquarius tape images and it came out as a working wav file that sounded like data, although I doubt the file would work as I think it merged the two channels as one, so hopefully it should be possible if I can find a conversion program that'll do separate channels and allow other tweaking.
I also tried importing it as raw data in Audacity and it came through, although it doesn't sound like anything I'd expect to be audio data (at least from my experience with TI 99/4a cassettes), so I'll need to do some serious playing around with settings before that has any hope of working. Unless anyone has suggestions?
I guess my immediate plan when I get the board is to build that circuit Charles MacDonald posted which goes between the deck and the PCBs and can be plugged into a PC sound card for dumping, and see if I can salvage the data off the original cassette, then take it from there. I still want to figure out have to convert cas files regardless as I want an Angler Dangler setup REALLY bad!