I think txp is talking about making reservations only (when holding a railpass), and not online ticketing.

I must admit, it is a pain. Apart from DB, for Italy you can book the combined supplement/reservation required for high-speed trains on the trenitalia website, and change it once free-of-charge. (Choose "global pass" in the dropdown listing other fares).

Otherwise, I resort to using an agent locally. Not really an option if you are based in the States, of course.