No, the OCP France is not valid to Brussels. 

You could buy a cheap ticket to Tourcoing or Mouscron frontière (whichever is cheaper) and then use the pass from there. It will be slower though.

Or you buy a TGV ticket to Lille-Europe, stay onboard the same train and use the pass from there (change seat/carriage).

French domestic TGV/Intercités reservations cost 10€ at first and then 20€.

Btw if booking well in advance (and depending on the travel date) a Brussels - Geneva ticket via Germany could be a lot cheaper. However it's slower and delays happen frequently (leave early).

Thank you for your answers @AnnaB @thibcabe 

Is there still no ticket with which you can buy a supplement, to still travel between Brussels and Lille at a kind of interrail rate?

I'm afraid not, but I would like to ask here anyway.

There isn't here* but if there were it would be a "discounted full-fare ticket" = advance tickets would still be cheaper.

If you can get a 25€ TGV ticket to Lille that's a fair price imo.

*it does exist on TGV/ICE between France and Germany

@AnnaB @thibcabe 

@rvdborgt confirm that this is indeed possible. So your information is not entirely correct. You'll know that for next time.

There's a Pass 2 reservation for the TGVs from/to Brussels, which allows to book a TGV with a pass only valid in France. The price is the same as the Pass 1 reservation. Only available by phone or at the ticket office.

https://public-pf.b-europe.com/en/Producten/Details?id=b4414e63-7282-42b6-8b7f-db33ff06cd25