Bob,
Is there anything in Simmons letters?
Firstly, and at the risk of you thinking I'm obsessed with maps, it's necessary I think to look at the "normal" route into Spain from Almeida and vice versa.
In my opinion, the journey Vale de Mula to Gallegos that you describe is the "normal" trans-frontier route between Almeida (via Fuerte de la Concepcion) and Cuidad Rodrigo (see Horward, "Pelet", p. 50)
So the prisoners, whoever they are, are being escorted not into French held, but Spanish held, territory.
Secondly, Mark Urban p. 52 "Rifles" suggests that, after the affair of Barba del Puerco, there was some suspicion that priests had perhaps been the source of French information supporting the possibility of a successful coup de main against the allied outposts.
So,perhaps, these priests (whoever they are) are being handed over to Spanish authorities - not French.
Conjecture, but plausible.
A