For the benefit of future searchers, the answer:

We went to the transfer desk before immigration, and they issued boarding passes for our Air Austral flights. They took our BA baggage tags, issued new ones and asked us to describe our baggage, so I assume that they collected them from the belt and rechecked them for us. We skipped the immigration queues and were ushered straight through to departures - so a definite advantage.

The Priority Pass lounge is fine - busy at times depending on the flights going out (we were there for six hours). Breakfast snacks, tea, coffee, free wifi. Nothing special but better than the terminal...