To check back in...
I tried out the poor man's manual machine, that is, I leaned against a wall (or lamp post), locked the rear brake, balanced the bike on the rear wheel, eased myself onto the pedals one at a time, then started balancing, keeping the rear brake locked.
While a very meditative way to pass the time while waiting before or after a ride (try easing backwards until you're sitting on the tire!), I don't think it's the same type of balancing as manuals. In a manual the rear wheel shoots out forwards or backwards from under you, but in a manual machine/rear wheel stall it's fixed in place, so you can push against it.
However, it will help you a lot with rear wheel trials hops where the brakes are locked! I landed a tire tap on a quarter pipe deck last week, after the "manual machine" practice, and I think it helped with that as I've been trying them for ages.
What might help is 1) remove the chain (to prevent pedal kickback/pressure) 2) lock the rear brake 3) lean against a wall/post and balance the bike on the rear wheel 4) get on 5) let off the rear brake. Though you'd be just as likely to loop out, and at that point you might as well just practice manuals normally?