You can now book pass reservations on www.raileurope.com. You have to add a rail pass, which will then be applied to all passengers in the group, and at check-out, you need to supply a pass number.
I did a test for Italy (Frecciarossa Milan-Rome) and there don't seem to be any extra booking fees. And you can select a seat for €2 extra, like on the Trenitalia website.
No luck for Spain yet. It will be interesting to see if it can book anything that can't be booked online elsewhere.