I did a recon ride up Grouse yesterday, so I’m going to answer my own question in case anyone else is interested in this.
The lower section is good, mostly hard packed with some marble sized loose pebbles, and bigger rocks imbedded in the road in spots - easily doable on a road bike. The upper portion on the ski resort is worse, with bigger, looser rocks but I was always able to pick a decent line through the bad spots, and managed to do the whole climb without getting bogged down or even having to put a foot down.
The descent is doable but sucks, mostly because you can’t just let it rip. I lost my patience a few times and got a pinch flat which served as a good reminder that I was still on a road bike. A cyclocross bike would have been nice for the bigger tires and better brakes. Some MTB skills are definitely an asset, but if you’re careful and patient, it shouldn’t be a problem.
All in all, this ride is not very pleasant, but doable. I’m going to go for the full triple crown later this summer, but I can’t see myself ever doing Grouse just for fun.