French railways have 2 prices for domestic Interrail reservations: 10 EUR (with limited availability) and 20 EUR. When the 10 EUR pass reservations are sold out, then pass 20 EUR reservations are available until the train is full. However, the Interrail reservation website has problems to show the 20 EUR reservations. The train you want to book now only has 20 EUR reservations and you can indeed still book them on the Belgian railways website:

There's a 4 EUR fee per order (which can include multiple persons and also a return) but you need a pass cover number to book. If you have a paper Interrail, you can find that on the pass cover; if you have a mobile Interrail, you can request one here.

Alternatively, you can order the reservation by phone from Belgian railways. There's no extra booking fee, but you'll have to pay for the phone call. No pass numbers are necessary and the reservation will be sent via mail.

Cover nr: on the old-style paper pass this NR is NOT on the cover, but on the actual printed ticket inside. I needed even 3 covers last time for a continuous 2 month pass-and even then I used 1 line for as many trains as I could fit in.

I also noted that since then all TGV that bypass Paris would cost at least 20€ as extra to pay, or more if quotum full. Probably

No, it is on the cover:

https://www.interrail.eu/content/eurail/en/help/reservations/where-can-i-find-my-paper-pass-cover-number

I just tested Lille - Lyon and I found plenty of 10 EUR reservations: 

Do you have any examples of trains bypassing Paris where the minimum reservation fee is 20 EUR?