I doubt it would be valid for Interrail reservations but you could always ask SNCF. Maybe someone else here knows for sure.
Even if it was valid you should bear in mind SNCF do not sell passholder reservations online so you would only be able to use ticket offices/machines or SNCF phone booking.
You would need to make many reservations to recoup the cost.
A Eurail pass is your ticket. On which you need to make seat reservations for trains with mandatory reservation (basically all SNCF TGV Inoui and Intercités services). The cost of these seat reservations are not subject to the carte avantage discount.
Carte Avantage only gives discount on normal tickets (and is very interesting for normal tickets)
Get your seat reservations here for domestic TGV services: https://travel.b-europe.com/Eurail-GE/en/booking-tgv#TravelWish