You could check renfe.com. Renfe don't have pass holder quota, so as long as regular tickets are available, pass holder reservations can also be booked. Check each train separately because Renfe doesn't handle journeys with a change very well.
Also be aware that there are multiple stations in Elche / Elx. 2 on the old line (in the city) and 1 on the high-speed line (Elche AV / Elx AV, several km outside the city).
I would like to book a train from Barcelona Sants to Elche on December 24. On the Renfe booking site, the 5:50 AM train is available, and I would prefer this option. However, this train does not appear on the Eurail reservation site.
If I book through the Renfe site, I will have to pay the full fare. Where should I make my reservation?
It's two trains with a change in Madrid from Madrid Atocha to Madrid Chamartín (there are frequent Cercanías trains between those 2 stations).
You can book on eurail.com. You have to book both trains separately: first Barcelona to Madrid, then Madrid to Elche.
2nd problem: no price is given for Madrid to Elche. Solution: book to the next station: Orihuela (price is the same for a pass reservation).
Also keep in mind that you're going to the station on the high-speed line, which is not in the city, so make sure you know how to get to where you want to be.
