Thanks for all the ideas here guys. Lots of food for thought.
Anchoring in a full scale ship does involve coming to an absolute stop, paying out the anchor and then gently reversing to let out more chain. Then generally after stopping letting out the chain, to carry on going astern to see that the anchor is not going to drag. The speed that the anchor hits the bottom is not important so, having brought the model ship to a stand still, one can pay out the chain until the anchor's on the ground and then reverse gently to pay out more chain.
I'd checked the links about working anchors and there were some good ideas in those which I had considered.
Ian, I like your drawings, but thinking it through further there might not be the need for an anchor winch as well. Why not let the chain drop into a chain locker after passing through the motorised rubber rollers. Only one needs to be powered, the other can be sprung against it, gently, to again allow for slippage should the chain catch or the anchor reach the hawse pipe.