Nope. It's true that during the high season reservations are compulsory on The GoldenPass and Glacier Express trains, which is why sometimes there is a supplemental cost. However, at this time of year reservations aren't necessary. Last November, even on weekends, the panoramic trains were about 1/4 full. (I think that's why they always have day cards on offer in November: it's a pretty slow month for train travel.)

If you look at the map I linked to in the original post (here is the link again: it's a pdf) it shows all the train lines in Switzerland you can ride on with a day card. Any train line in solid red is 100% covered by the half-price card + day card. Any train line in dotted red is a line where you get a 50% reduction. So you can get to Zermatt and back with a day card, but to go up to Gornergrat you would have to but a ticket at 50% of the full price. (Note: yellow lines are bus routes).

Last year I took the Glacier Express trains to Zermatt, and on a separate day from Brig to Andermatt using these 39.- CHF day cards no problem. One nice thing about the trains being so empty was that I could change around where I was sitting depending on where the views views were. This weekend I'm planning on doing the GoldenPass route.

So no excuses, there are no supplements to pay. Get out there and travel, it's going to be a (sort of) nice weekend! (Bring your mittens!)