You should be able to buy a reservation without validating your railpass. I don’t recall ever being asked to show the railpass when buying a reservation. Locals sometimes have an open ticket or some kind of pass and need a reservation, so selling “just a reservation” is not uncommon.

Showing your railpass does make it a whole lot easier to get your point across if you’re having a language problem, but even in that case, the reservations agent shouldn’t care whether it’s validated or not — although they’ll probably tell you where to go to have it validated.