A custom-made cassette reading peripheral card in every slot, and eight cassette players running in parallel at high speed?
Dealing with sync issues would be top of the list.
Maybe stagger them, so they can be stopped every so often. Reach key data, kill motor, wait, start motor?
Or maybe just toggle the motor line quick? Just enough to slow it a little!
All kinds of insanity possible.
Stereo tapes could possibly deliver 2400 baud x 2.
Eight 4,800 baud tapes * 2 minutes 'ish
I am coming up with 0.5 to 1MB range numbers :(
But I really like this craziness!