These reservations are not possible online because the ever so customer-friendly Renfe don't want that.

A list of known ways to book is in this post:

I’m traveling this exact route from Barcelona to Seville on Nov. 30, 2023 and I think something may have changed recently on the Eurail site, as I’m seeing seat reservations showing as being available for 1st and 2nd Class for dates that are less 90 days from the inquiry date.  I’m going to check back on Aug. 30 and hopefully seat reservations will be available for my journey.  Hurray!  I believe the link above (“How to get Reservations”) does say that AVE trains can be reserved on the Eurail site. 

Yes they are now available on the website !

90 days in advance is not always true (especially with RENFE) so you may have to wait a bit longer

It’s good to see AVE and ALV seat reservations available on the Eurail site. I’m planning my connection from Madrid Charmartin to San Sebastian/Donestia (in December 2023, so its way early, today being Aug. 6).  I researched this route for dates over the next few weeks to get a sense of what might be available in Dec. and I’m finding that AVE and ALV/Alvia seat reservations can be purchased on the Eurail site, but the Intercity (Spain IC), AVN/Avant and TRN lines are not available on Eurail. Madrid to San Sebastian has various times for all of the foregoing lines with Intercity and Alvia being direct.  I’m wondering if I should just wait until I arrive in Spain on Nov 24 to buy a direct Madrid-San Sebastian ticket for Dec. 8.   The Eurail site says these tickets can be purchased at a “Main Station”, but based on what I’ve been reading on this platform, there are very specific ways (which I’m in the process if figuring out) to purchase tickets for Spanish trains.  I suppose I’ll try “Main Stations” when I arrive in Europe in October and if all fails, I’ll buy Intercity or Alvia when I get to Spain.  

24 November should be OK. You can also buy Spanish reservations at the ticket office in some other countries, like Germany and France. Here's a list of known ways to buy reservations for domestic journeys in Spain:

Thank you, @rvdborgt .  Very helpful information and it’s good to know that Nov. 24 is probably safe for reserving a Dec. 8 Madrid to San Sebastián train (but I will try to reserve earlier in Germany).  Recently, I read on the Seat 61 site, which I have found very informative, that a rail expert can arrange pass holder reservations in Spain, but they are hard copy and can only be sent to UK or collected from ticket in Germany.  It looks like the reservation systems change frequently and are evolving, so it is best to get information from multiple sources just in case.   

AFAIK that's a UK travel agent who uses the DB system for these reservations. If you're going to Germany anyway, then you can buy from DB directly without any booking fees.

Perfect, thanks again @rvdborgt.  I’ll book in Munich in October at DB.