AFAIK this is not possible.

Here's an overview of known ways to book reservations for Spanish domestic trains:

And here for the TGV/AVE between Spain and France:

Thank you for the quick reply rvdborgt! This proviso should be in bold somewhere since it seriously hampers the use of the pass. Eurail makes it sound deceptively easy to use the pass when, in reality, you need to get into Spain and set aside dedicated time to get longer trips set up, potentially even in the off seasons. 

When INside ESpana the really only way to make that RES-which is about always needed, is to go in the line and wait turn and then approach counter-hope they can understand /en/ or your mangled attempt at espanol-and thats it. RENFE does not like passholders and does it utmost best to discourage trips on them as much as they possibly can. In general you will also need to show your pass is valid-also for that day. IF you hit a nice employer take the chance and try to make all you want in same time. Which of course denies any spontaneous change of plans.

(though now as I write this I recall that machines should be able to change RES-at least they can for AVE and normal tickets-never seen/heard about this being possible with only RES on passes)

Estaciones off the main touristy places are usually far less busy-and may still even have a kind of curiosity that this funny estranjero turista may need help.

The only system that still allows passholders to do it for RES only is now FR/SNCF-with a very long list to wade through before you hit it. In DE=Germany anyone can use machines for a RES-only-but that is not tied to tix at all there.

Thank you for the detailed info mcadv. Hopefully this helps others avoid this problem. It seems like Passes shouldn’t be used in Spain for the time being. 

If you are in Spain you can call RENFE as described in the link in the reply from @rvdborgt . You make the reservation by phone and then collect the reservations within 72 hours at a railway station.