Seat reservations
To clarify, it is the Eurail Pass that is not valid on the Glacier Express, but the Swiss Pass is valid, as confirmed earlier. The Eurail Pass (as well as the Swiss Pass) is, however, valid on another scenic Swiss trip, the Bernina Express on the Rhaetian Railway.
In 2005, I had some difficulty in getting seat reservations for the Glacier Express with my Swiss Pass. I did not want to wait until arrival in Switzerland, as it was mid-summer. So, about 45 days prior (on another trip to Europe) I stood in line at two ticket offices in London, who were unable to help, and then again at the international desk at Gare du Nord in Paris, also unable to comply with what seemed to the ticket clerk as an exotic request ("why not get them in Switzerland?"). I finally emailed the ticket office in Zermatt, and they were able to make two seat reservations, which were held for me until arrival in Zermatt, and were charged to my credit card, which I emailed them at the time, as inadvisable as that may seem.
Perhaps, per the email quoted earlier, things have improved. In any case, there is a person in the email to contact if all else fails.