Ahhhh thats a shame, I only just found the Arduino and it looked pretty exciting with all the possibilities! I think that would be the best way of implememnting keyboard control too, though all the tutorials I read were based on inplementing a keyboard matrix for a keypad or QWERTY keyboard rather than anything musical.
I had an idea of meeting halfway with the optigan and the mellotron, and instead of using cassette, use the tape, but wrapped round the circumference of a disc, say 3 inches in diameter. Drive this directly with a motor, no gears, wheels, belts etc. The tape head could be mounted next to the disc and then the speed of the 'tape disc' is adjusted with PWM. I've been pondering the possibility of (somehow) getting my hands on an old DD walkman which used disc drive as opposed to belts.
A few people have had success controlling motors with Arduino and PWM, as per Youtube, but my only concern is whether that will stretch over 2 octaves. The whole transport needs to do nothing but play back, therefore the pause, stop, RW/FF functions are all irrelevant, so I reckon the system could be simplified. I've even thought about using one tape per octave, but that would complicate things where I'm striving to simplify
I do actually have a dead Tascam 4 track that needs repairing. I'm not sure exactly what went wrong with that, I think the motor is dead so I can't test if the head is functioning. With a 4 track head, using a single cassette, you could then (somehow - this is where I get lost) configure the 2 octave keyboard to have a track per 6 semitones. Each track of the cassette could be a long sustained note at every half an octave. So it'd have to be something like Trk1=C2, Trk2=F#2, Trk3=C3, Trk4=F#3. Then the motor needs only know how to swap between 7 speeds (to cater for that elusive 25th key, which would be C4 in this case.)
I know that the actual melloman idea works great using 13 walkmans, but everytime I come back to that idea I think 'OK, where does one purchase 13 identical, high quality walkmans for next to nothing?'. If I answer that question, I might have a starting point