Hi Alistair,
Seems like you stirred quite the interest! It's a very novel idea and also very cool. The comparison shows just how similar they two are, but this is of course bearing in mind a 90mm refractor vs a 203mm f/4 newt - I suspect the newt might always win
This sort of set up seems very well suited to fast newts, since the DSLRs obviously dont have peltier cooling for long subs.
I had no idea the D3200 had 60% QE, if the bayer matrix is easy to remove it might be worthwhile pursuing that path rather than the canons? I'd be very tempted to. Question then becomes how to use NB filters; you would have to get a M42 extension tube from the T-ring (Atleast 40mm to push the FW away from the DSLR where the prism sits), and if you are using a OAG that equates to almost 90mm backfocus! The steep f/4 light cone may result in a smaller-than-the-sensor image on the guidecam since the pick-off mirror is almost the same size as the guidecam sensor (I'm not sure if this is going to happen but food for thought).
I wish you many clear nights, can't wait to see the outcome of this project!