the other day while spraying the plastic kayak with mcnett uv protectant. i noticed a couple of the thick nylon straps that hold the hatches down were really stiff. A quick shot of mcnett on them and they were instantly new and supple. (remember the old tarn-x commercial?)
I've got a couple old Quetico Superior #4 packs purchased used from an outfitter. Some of the straps, and the bags themselves are also stiffer than when first purchased. Thus far they've only been hosed down before and after use. Maybe some warm soapy water?
Question is: What could a few shots of uv protectant hurt? The stuff doesn't seem greasy like it would attract dust and dirt. Do you do anything to protect or treat your synthetic packs?

I have a 40-year old nylon backpack that's still in great shape, and I've never sprayed it with anything. (It's an antique external frame - the retro look gets lots of comments!).

My nylon canoe pack is much younger, and also isn't showing any real signs of wear (or UV damage).

The UV protectant probably won't hurt things, but it might not help either. It could make your pack more slippery (sort of like putting Armor-All on your car's steering wheel, not a good idea). I like to adjust trim on my canoe by moving the position of my pack forward and backward, so for me, a more slippery pack that shifts position when I don't want it to would not be a desirable outcome.

If it ain't broke, don't fix it?

I've sprayed black pack straps and my yellow PFD. They were both noticeably faded and I figured it couldn't hurt. I don't recall any bad effects. Nylon on nylon is pretty slippery anyway. I don't recall if I used 303 or McNett - UV.... I'd guess McNett. I have nothing to compare though so....

"What could a few shots of uv protectant hurt?"

who knows? if you do this, just pick a small "test" area and give it several days to see if there is any bad effect.  I'm thinking particulary if the pack is coated nylon, you may dissolve the coating.