followup:
it seems that the rental car counters at BOD airport (Aéroport de Bordeaux - Mérignac) are open on this particular holiday.
I've resigned myself to skipping the TGV (sniff, sniff) and taking AF instead. we'll save a couple of hours each way by my estimates.
It shouldn't be too much to circumnavigate Bordeaux from the airport out to N89 eastbound, right?
merci