I have a similar challenge for our MRC layout Okehampton 7mm (OK not GWR), the Meldon Quarry engineers coach was un-coupled near its siding in Okehampton and a type of air motor was used to send the coach down into its siding. there is a gradient drop to the buffers on the real thing.
I have thought of having a working braking system on the coach.
Motorising the chassis.
mini bearings for free running
my best thoughts are, is to use a small coreless motor in place of the dynamo to a pully on the coach wheel. and mini bearings on all axles.
Going back to the model slip coach idea, if my memory serves me right, the coupling hook was a disk with a 2 slots cut out one for the coupling the other for the release mechanism.
Using a servo this can be achieved, and a read switch to activate it or dcc.
Ian G