I hate to admit it but Rail Europe is quite convenient for making reservations only for those who already have a rail pass. But their reservations are way overpriced and cannot be changed once paid for.
Is there another way to make reservations online? I know you can always make them at the train station, but lines can be quite big at times and the online convenience would be great, if possible. I know DB offers online reservations but only for travel within Germany. None of the other railroads do, to my knowledge.
DB, in all likelihood, will make seat reservations for you anywhere in Europe, but there are limitations about what can be done on line for travel outside Germany. You can call them up, for example to get help with more options beyond what it offered on line.
For Frankfurt-Paris, for example, because there are limitations on the number of seat reservations allocated to passholders, this may be a function that is beyond the capabilities of the current on-line system. Otherwise, getting solely seat reservations on line will depend on the capabilities of each operator's indivdual website (Italy, apparently yes; France and Spain, apparently no; others, unknown). As can be imagined, selling this specialized type of reservation may not be the highest priority for some systems. And be wary: passholder fares on Thalys and Eurostar may cost more than simply buying a complete single ticket far in advance.