Has anyone ever seen and used a Royalex Sundowner 17 with Bucket seats and a foot brace riveted through the hull? I’m curious about the durability of rivets through Royalex

I wouldn't worry about it. I've had rivets through one of my fiberglass hulls for foot braces for 37 years with no problems. Should be even more secure with Royalex. It's pretty standard practice to drill holes in Royalex just under the gunnels for lacing up float bags. For the seat, I'd inspect for any cracks. If stored outside in lots of cold weather, Mad River had some problems with cracking Royalex associated with screwed-on wood gunnels. I've not seen this with mine so far (1990s) even stored in sub-zero weather.