TL;DR: The pavement condition is good overall.
I drove the western 15 miles of the course the day before, getting out and walking or running in about half a dozen places. Not encountering any bad areas, I decided to go for it.
Amusingly, Main Street in Hopkinton has been recently repaved with a beautiful fresh coat of blacktop... up to about 100 yards beyond the start line. Beyond that, the asphalt is generally in good condition, fairly smooth though by no means new. Maybe 15 years old if I had to guess. In the first couple miles there is a 3-foot wide longitudinal patch (likely for utility work) that aerial views show was done in 2013/2014. I found this area to be a bit smoother. Pavement through the towns was generally good. Some areas of Natick I found a little rougher, such as along Fisk Pond.
The eastern half of the course was more inconsistent. While still mostly good, there were stretches where the asphalt was rougher. These were usually not too long. Parts of Newton, such as the downhill after Heartbreak, were in worse shape than others. As I’d noted in my original post, some areas of Brookline such as western Beacon Street were probably the worst. On Street View, you can clearly see areas of cracking and raveling; this is from about mile 22.5 to 23.5. It was still manageable—I did my best Ken Bob imitation—but I’m glad I had gotten in some training on more-worn surfaces. I’ll also mention that my soles were becoming more sensitive after about mile 20, so part of my perception may be because of that.
I heard that the course had been swept a few days before the race. There were no leaves or twigs, and very few pebbles.
I didn't see any other shoeless runners. At about mile 15, someone called out that I was the first barefooter they'd seen. So unless someone puts forth evidence to the contrary, I'm going to just lay claim to first place in the barefoot category with a 3:29