Brugge to Paris

For purely domestic trains within Belgium, there are no reserved seats. The fastest route from Brugge to Paris involves hopping on a domestic Belgian rail (NMBS) train to Brussel Zuid/Midi station, and from there, taking the high-speed Thalys service to Paris. But with a railpass, Thalys requires you to purchase a hefty supplement, not simply a reservation. This is just one among the dozens of reasons why most of us here consider 3rd party rail passes a waste of money.

I'm not sure how the Brugge to Paris via Lille option would work, other than not needing a reservation to travel to a border station.

Brugge to Paris via Lille is a reasonable route, you may sometimes have to change at Kortrijk as well between Brussels and Lille. There are two potential warnings:

Firstly, although there is no discount for advanced purchase between Brugge and Lille, the journey from Lille to Paris will be much cheaper if you book in advance. These advanced tickets are limited to specific trains, however, and you MUST catch that one.

Related to the above, the local trains from Brugge to Lille arrive at the old Lille Flandres station. Some of the trains to Paris leave from there, but others run from the Lille Europe station on the high-speed line. The distance between the two is walkable, and there's a tram if you have heavy luggage, but make sure that if you book a train from Lille Europe to Paris you leave plenty of time to travel between the two stations.

I agree that it is false economy to go to Lille. The Brugge-Brux/Midi-Paris route is faster and discounted if bought in advance. Lille does have a good art museum. I believe the two Lille stations are internally connected and no worse than a looong London Underground connection.

Thanks for your helpful comments. After doing more research I realize that it is "false economy" to try the Lille route.