Having visited some of these peaks over Memorial Day Weekend, I was able to find out first hand how drivable these roads are for small cars.
For Gold Brook/Beaudry Road, it was in rough shape, with a fair amount of pot holes, with some washboard and protruding rocks to watch out for. There was even one rough muddy spot where my poor car bottomed out (twice). It was a rough enough ride that once I reached the turn for the Kibby Mountain trail, I simply parked on the side of the main road and walked the last 0.3 mile. I found the logging road to be rocky, but without anything protruding or too large, it's very driveable. Although unsigned, the trail comes in where the logging road's shoulders widen enough to accomodate a parking area. I even observed a Corolla parked there, which just goes to show that even in a small car, careful driving can get you all the way to that trailhead. However, during my hike I ran into the owner, and we both agreed that Beaudry Road was in rough enough shape that we wouldn't willingly drive on it again unless the road had been improved.
After Kibby I continued on to West Caribou, and en route I found that Beaudry Road was in better shape during that last 3-4 miles than for the first nine in getting to Kibby. Once reaching West Branch Road, I found it did live up to its description of being narrow. Really narrow. It was basically two ruts with a grassy strip down the middle. I drove about a half mile down it, but I was getting really paranoid that my oil pan was about to make contact with a rock hiding in the grass. Walking the rest of the way in, I observed that despite its narrowness, the road was in solid condition, and the few protruding rocks were obvious, and there was a rough patch or two. About two miles in there's a bridge where part of the road is washed away at the approach, so one needs some boards or a higher vehicle in order to drive onto it. At three miles, I observed that some people had driven in their pick up that far and were camping near the twisted bridge, which shows that a higher clearance vehicle can still make it at least that far. My car could have probably made it the first two miles, but even though the road seemed okay, it was so narrow that it would make me nervous enough that I'd rather just hike in.
The next day I visited Boundary Bald, and the main dirt road going in (Bald Mountain Road) was in adequate shape (just a few rough spots and some easily avoidable rocks to watch out for). However, when I reached Notch Road, I found that it was rather rocky and rough, and considering I'd only be on it for 0.2 miles it just wasn't worth it in my low car. Not for gaining so little distance. Higher clearance vehicles shouldn't have a problem getting to the turn for Trail Road.
Later that day I drifted down to Coburn. Like everyone else, I simply drove the first 0.25 and parked a little before the first stream crossing. A sturdy truck can make it all the way to the base of the mountain (as I observed in person while I was there).
On the following day I visited Moxie Mountain. For some reason, Troutdale/Town Line Road was pot hole city when approaching from the north. Once on Dimmick Pond Road, I drove with great care and was able to get my low car all the way to the campsite area. Along the way I think I found the washout that Papa Bear mentioned in his trip report. When I was there, water covered at least four fifth's of the road surface. However, in examining the area, I found that the puddle wasn't that deep, and there weren't any rocks lurking underneath. Sticking to the north, drier side of the road, I passed through both times without a problem.
Just overall, it seemed like all the dirt roads I encountered that weekend were surprisingly rough. I can only guess that the recent rains had prevented the roads from being graded since mud season. I just hope that anyone who visits these mountains in a low clearance vehicle finds these roads in better shape than I did.