Yep, no easy way to do that. Mostly you can just take the public fare for the most flexible ticket from each railway's website and divide by 2 (or 4 for France etc. etc.) and you are more or less there. ATOC produce a guide to FIP travel on globally priced trains on their website, which is useful, although doesn't have all options/possibilities on it.
don't DB part own Thalys?
DB's booking centre for RSTL still offer FIP fares on Thalys and the Rail Europe shop opp. the Hauptbahnhof at Cologne can as well. But both SNCB/NMBS and NS don't seem to have any availability within a week or two of travel.