Renfe has just opened regular ticket sales for the reintroduced AVE services between Barcelona and Lyon and Madrid and Marseille. I have called their customer service and I've been told interrail is valid on those for €10 in Elige (standard class) and €13 in Comfort (1st class), and that I could book these reservations online. The latter I've not been able to find out how, but hopefully the first statement is correct.
I suspect these reservations can be made through Renfe's ticket office. Possibly later also through DB (and other channels with access to Renfe's system).
Currently tickets on the sections Valence-Nimes-Barcelona-Madrid are on regular sale for just €19, and from Lyon/Avignon/Marseille for €29, so it's probably not really worth to use a travel day for that (unless you have a continuous pass or more travel plans in the same day).