I am envisioning a low to zero support brevet up the Fraser Canyon to Lillooet, then over the mountains to Pemberton, returning on the Sea to Sky.

It's a hard 600, with over 9000 meters of climbing. The difficulty in getting to Pemberton should not be underestimated. The Fraser Canyon is hot; the Duffey Lake road is high and cold, and there are grizzly bears up there. After that, you have to get from Pemberton to Whistler, which is also hard. :-)

But the parts of the road around Lillooet are really special. The high mountains around Duffey are gorgeous, and the twisty Euro-ish descent to Pemberton had me absolutely cackling last time we passed that way. This is a really nice route. If you relax your expectations of finishing in the time limit and resolve to accept an Hors Delai with pride if it comes to that, it doesn't even have to be that stressful.

This brevet is sufficient for partial qualification for a Randonneur 10000 award, since it is well over 8000 meters of  climbing. https://www.audax-club-parisien.com/en/our-organizations/randneur-10000-en/