allow the client to pick a specific relay for its registration

Currently the flashproxy-client will ignore the Bridge line in torrc and connect to whatever relay the facilitator chooses to give it. The means that the client has no control over which relay it connects to; and the facilitator has full control.

Possibly you avoided this design on purpose, but we could give this control back to the client - we can extend the client registration to support an optional explicit relay, and the facilitator will tell the proxy to connect to this relay. (If no relay is given, the facilitator falls-back to picking one of the relays it knows about, i.e. the current behaviour.)