I think this was posted before, but I can't find the thread.
grazie!
I called the Gallery from the hotel in Rome the day before I went. When I arrived at the Gallery, I showed my pass as I paid for my ticket.
I don't know about reservations online - but I'm sure someone will respond here.
Recent reports suggest that you can no longer book just a reservation online--you have to purchase the full ticket, which obviously defeats the point of the Roma Pass.
To make just a reservation, call the Gallery or better yet, ask your hotel to do it for you.
We'll be in Rome the first part of October. Last week I called the number posted on the website, pressed #2 for English and said I wanted a reservation, but was getting the Roma Pass. They took my last name, gave me a # to show at the ticket counter and apparently, that was that. There are several timeslots at 2-hour intervals you reserve.
http://www.ticketeria.it gives you all the info.