Expect it to be full-for you. These trains are in such heavy demand due to non-imagination of about all newbee passholders, that the small quota for them is exhausted in main season weeks before. Do you know about 14/7 in FR?

Plan B: there are a few 1000 posts here thus about very same-can do same trip with a few more changes for much less - and also much more chance on finding seats in TGV, domestic, change into free TER+rodalies.

ONly feasible way to find out exactly is call SNCF -use /en/ channel-and perhaps expect yet same agent who is sick of helping passholders.

Also check normal advance fares, as I underst you have not yet decided on pass: SNCF advertsies anywhere in FR on all busstops there are plenty of seats for/from 49€=full fare. The suplmt alone on a pass is 34/35.

IF you intend to travel further into ESpana too: think twice about pass-the very worst country for passholders. seat61.com has also plenty of pro-train info on howtodoit without pass.

Thanks.  I think you are right that it is unlkely.  Looking at that train, they have stopped taking bookings to Perpignan even though it stops there, it seems like it is pretty full.  Sadly the fares are nothing like €49 but there we are!

You can call SNCF and make a reservation if there are seats avaliable. No need of Pass Cover Number. 

https://www.sncf.com/en/customer-service/contact-us/telephone

 Press #85 for English, no booking fees, reservations are sent via e-mail

Many thanks