If I keep cranking on those allen head bolts and adding washer weight, I wonder if I could drop the Cheetahs bite down to 100 RPM? It makes me wonder.
You could just use a freewheel motor output sprocket if you can find one strong enough, that's what I use on my 50V 1860W shifting e-bikes, or in this case an e-Beast.
Nowhere near the power you are talking but it has gears so it might run with you.
I was thinking an easy mechanical disengage of the motor from the drive train.
That would be pretty easy to do with a belt but a chain presents a different problem and a cent clutch seems like a poor solution.
How often do you plan to be pedaling it anyway?
Speaking of braking by motor drag, have you considered a regenerative braking system?
That big electric motor is a heck of a generator, have it charge your batteries.
If you think that motor has drag doing nothing, it would probably lock your back wheel even hurtling downhill even with all that weight on the back wheel if you tried to charge with too much current, but one thing is for sure, it would make for a good brake if you could control it ;-}