If you have two gatekeeper trunks, one in a G711 region and the other in a G729 region and you setup the Route list with G711 as the first priority and G729 as the second priority and the gatekeeper does not have enough bandwidth to process a G711 call, then it will fallback and use the G729 trunk to process the call.
You do not need to enable BRQ to make this happen.
CCIE Voice notes 