Ah, the good ol' sitting glissade. I remember a Rainier climb back in 1986 or so when we had a glissade good for about 3,000 vertical feet. Wow, that was a fast descent.
But of course, it was also murder on pants, especially in "corn-snow" conditions when you were basically sliding on little ice crystals. Once upon a time that was less of an issue because the only waterproof pants available were inexpensive coated-nylon pants. So if you wore a hole in the butt, it was no big deal. But when you pay $300 for a pair of high-end Gore-Tex pants, you probably don't want to trash them.
To be honest, though, I don't know of anyone that makes a pair of good Gore-Tex pants that have a big Cordura patch on the butt, which is what you need. That would add complexity, stitching, and taping issues. But even if some GT pants had a reinforced patch, I wouldn't slide on them as you'd at the very least wear off the water-resistant coating, limiting their effectiveness. So, your options aren't that great. Sliding on your emergency blanket is certainly one, as is packing a pair of cheap coated-nylon pants to sacrifice for your descent (like the Ibex Pants from Columbia Sportswear: $18; www.columbiasportswear.com).
While I don't want to preach, you might also consider working on your standing glissade. Safer, reallythere have been lots of cases where sitting glissaders tumbled forwards and whacked into trees or rocks. Not quite as much fun as that long, sitting slide, but equally satisfying.