You can't choose seats but you can give some preferences on www.raileurope.com for some trains.

Added advantage is that it's cheaper than via Eurail, since Raileurope don't charge extra booking fees. Click on "Add rail pass” and select your pass to book pass reservations.

FYI, the rail planner app can't book anything. It just forwards to some of the websites where you can book.

For Eurostar from/to London, regardless of where you've booked, you can change your seats and pick new ones from a map on the Eurostar website, using Manage Booking. Not sure if this has also been implemented now for their continental routes (such as Rotterdam-Paris), but you can try. In any case, neither B-Europe nor Rail Europe offer any seating choices for Eurostar.

For the TGVs, you can give preferences on Rail Europe.

Eurostar and the TGV Marseille-Brussels have a limited number of pass holder seats, so don't wait too long.

Thank you for the information!