[flal_]

You van already do Bruxelles to Perpignan with a direct ( french ) TGV train, in 6/7 hours. If you want to go to Spain I believe bruxelles Barcelona via Paris is also quite practical. Everything is much pricier than plane and no night train though

[twic]

I've done Paris to Barcelona on a sleeper to Perpignan and an express from there. I think for people who like sleeper trains, a through service would be far more convenient, even if it stopped in Perpignan early in the morning.

I suspect there is not a lot of track capacity across the Pyrenees, though, which would mean the sleeper train would have to get in before the morning expresses start.