(There is no section O for anybody who actually reads all of these, including me years from now.)
I solved my parking dilemma by finding a small parking area for Bradley Palmer State Park along Highland Road in Hamilton, then hiking that road to Cutler Road, then down to the trailhead. It was a 45 minute hike, but not unpleasant, with horses and cornfields to look at and a cool breeze at one point. There was some rain, which I countered by putting my raincoat hood on my head and holding the body around me like a cape so I wouldn't overheat or have to stop and take the camelbak off. When it wasn't raining, I carried the coat or tied it around my waist - not a fashion statement, but I didn't particularly care today.
The BCT section of the day started in Harvard Forest. I have to say that after the rain we've been having, I might have been better served wearing my wellies instead of sneakers, except it is possible some of the sections of deep muddy trail might have tried to suck the boots off. I sort of rinsed my sneakers in one of the last mud puddles in that section and a lot of the mud fell off before I got back to the car. The laces may be permanently brown. Also, mosquitos are feeding, not just buzzing around investigating hikers, so bringing the bug spray was a good idea. This piece of woods is not my favorite, but I got through.
Pingree Reservation had the same mosquitos and some of the mud issues, but on the whole was a more pleasant walk and I could see wandering in there again some time, maybe when it hadn't been raining so much. I saw a variety of horse jumps in there.
When I got back to Highland Rd, I decided to continue following the BCT until I got closer to where I had left the car. Perhaps predictably since I had not printed out a Bradley Palmer map, I got a little lost when trying to find the right place to turn for the car, but I did have my BCT map to get a safety bearing and walked out a paved road to nearly the right spot, overshooting a little. I recognized where I came out from my earlier walk to the trailhead and it was only a couple of minutes down a pleasant path to get back to the car.
The BCT portion was about 1.8 miles. The road portion was probably about 2 miles. The next section traverses Bradley Palmer and looks to be 2.5 miles on what I hope are clearly marked trails. Since I did section S earlier, this would complete the "East-West" part of this Map and I could continue the "North to South" bit. Just need to decide which end of the hike to park at.