Since you seem to live in Belgium: go to an NMBS international ticket office or call the NMBS (although their phone number isn't exactly cheap).

OR-depending on where you live also, book from LIlle and go there on normal train-often a new set is coupled there too=more chance on seats. Probably cheaper too-domestic price. The you should also be able to call SNCF and have it done via the fone.

Same problem but I am currently in Sweden so cannot go to a ticket office in Belgium. I have a lot of problems with double-checking seat availability information from sncf-connect, but on thetrainline.com I can see all days have seats left in second class. Interrail seat reservations only shows availability on one day. It shows the trains for the other days, but claims no seats are left for reservation. I am booking for two people simultaneously if that matters.

Absolutely no problem on the b-europe site for Lille to Marseilles on 27th Dec - both direct and via Paris.

Not sure whether any other site than IR can sell reservations only for Bru-Mars, but they are showing availability for the 0635 train.

The route I am interested in is Dec 30th Bruxelles 13:17 → Marseille 18:44. It seems I can buy the regular tickets for Bruxelles → Marseille, Bruxelles → Lille, or Lille → Marseille on this train. But I cannot buy the seat reservation from Bruxelles to Marseille through interrail.eu.

It is possible there are no more pass holder seats for that train. The TGVs from Brussels to the south of France only have a very limited number of pass holder seats available.

To be sure, call SNCF (press #85 for English) or NMBS.

There may still be pass holder seats for Lille - Marseille on the same train. On domestic routes in France, they are available until the train is full. On the TGVs from Brussels, they sell out before the train is full. In that case, you could buy a normal ticket Brussels - Lille (available from around 20€) and a pass holder seats Lille - Marseille (available from 10€).

You seem to be right. I can book passholder reservations for Lille → Marseille, and even Brussels → Lille (at a price of EUR 22, higher than regular tickets). But not Brussels → Marseille. I will try to call them.

I called SNCF and they said Bruxelles → Marseille for passholders was sold-out on their side. And they cannot do the Bruxelles → Lille passholder reservations either. And sncf-connect is crashing all the time. Maybe NMBS would have been able to help me, who knows. I gave up on hustling trying to get the cheapest deal. Just paid the fee twice by splitting at Lille with interrail.eu and got the tickets. Luckily got seats in the same carriage. Switching with luggage will not be too much of a hassle.

Please note, that sometimes SNCF Call Center can’t book reservations for pass holder. Only some people at the call canter can do it. Also do not call in french but only in English, because the probability is higher to get a reservation. 

I pressed #85 to get the English service, then asked to speak French saying I had asked for English by mistake, which the person on the other side (French speaker) did happily. In this scenario, what are the odds to get someone that can book reservations, and that will not lie about it?

They said something about seeing a “zero” on their UI, so that no reservations could be given. They even went so far as to say that passholder reservations were “forbidden” on that train. But not on the 06:30AMish train. They told me that if I could bypass this restriction by leg-splitting, then I should do it because they could not help me more, and the regular tickets are a bit more than EUR 150 a piece.

Ok, this would probably mean it is sold out. Because people that can't do it, they say it is not possible and not there are 0 left

other possibility is to book via b-europe the TGV Brussel to CDG Airport (if the train stops there and if seats are left for pass holder) and a reservation via travel.b-europe for the same train from CDG Airport to Marseilles. This would cost more, because you pay the reservation double, but would be online possible. You will have probably to change seats.

TGV Brusel (only CDG and Disney), Thalys and Eurostar

https://www.b-europe.com/EN/Kopen/Pass#TravelWish

All TGVs in France

https://travel.b-europe.com/Eurail-GE/en/booking-tgv#TravelWish

Ah. Angelo, just to disambiguate, I already got tickets by splitting at Lille (instead of CDG as you suggest) and paying the fee twice. I continue this discussion just by curiosity, to understand how broken the seat reservation system is for TGVs, in particular the ones crossing the belgian border.

I also tried to book regular tickets for Brussels → Lille (EUR 19 instead of EUR 22) through the NMBS app, but repeatedly got an “unexpected error” for Dec 30th. I could see the trains and prices if I looked in November though.