Same goes for Pamplona-Madrid Puerta de Atocha by the way! So the general question becomes: what to do when you have to book in advance but cannot book in advance??

There is a euskotren train from Hendaye station to Spain but it is not part of Interrail, but Tickets are cheap. It is a local train. 

Trains in Spain, can be booked via Deutsche Bahn Ticket offices. You have to provide train number and hour. They have to book without timetable. It is a bit complicated, but possible. 

RENFE runs these trains-and FULL mostly means the last section into MAd-or other main town-not between those minor 2 stops very far away from there. For that you can-in most cases-and assuming the RENFE ticketseller in his/her chair at counter is willing- do that till departure. But the other thing is, I fairly often get the impression they are unable or unwilling or ignorant on how to do it.

Elluke trein hep volle en legere stukkies. Is dat logies of is dat algebra?

Maar zeker in de zomer en in ESpana is het altijd zeer slim een plan B achter de hand te hebben. In ES is dat is bijna altijd de BUS.

You can walk from Hendaye to Irun - it’s a nice feeling to walk across the pedestrian bridge crossing the river and crossing from France to Spain. But there is also the small Euskotren which you can take. Depending on timings (and the fact that you have to pay a few Euros for this local train, you could get it to San Sebastian and get the Renfe train from there maybe. San Sebastian is a nicer place to change if you have a few hours.

Regarding booking reservations on Renfe trains, you can I believe book them by telephone and they hold the reservation for a few days before you pick them up at a station