When I was there I was all full of beans and in a royalex boat, so my plan was to stay on the water and wade/bash/stomp/line as required. Also, I saw another party on Black Pond who had been unable to find the portage trail, so I figured it wouldn't be obvious. All that said, the flow rate was pretty high for summer (seemed to me at the time), and it was some tough sledding, so if I had it to do over again I'd look for the trail.
Funny, I stopped at the campsite Sweeper mentions to look for a makeshift pole (didn't find anything suitable), but I didn't know that was the start of the portage trail! I was using the National Geographic Trails Illustrated map (Allagash Wilderness Waterway South), which is a nice scale but a bit of a cartoon, and it didn't show the trail extending that far downstream. I did eventually pick up a trail on river right about 100 yards downstream from the big drop. I imagine it's more often used in the downstream direction by people who don't want to run that drop. I attached a doodle on a Google Earth image. Red dots are my approximate route, but my advice is don't follow me, follow Sweeper.
Re: fees, when I came to the checkpoint I explained what I was doing to the gal on duty. The fees are the same for AWW and PRC, but she actually wrote down something slightly different than my actual itinerary so she'd only have to fill out two forms and not three. She did say she wanted to make sure PRC got their fair share -- apparently they're the poor cousin of AWW. I actually spent one night in a NMW campsite, which probably means I was a rogue camper, but no black helicopter came. It's all by the book up there!
In addition to the trailhead campsite there is also one where Scott Brook enters Black Pond, and that wasn't on either of my maps.
Chesuncook Village would be a potential starting point. I don't know what the parking situation is there as I've only paddled by.
Anyway, I never wrote up a trip report, partly because my camera was misbehaving so I have very few pictures. However, the trip was memorable so I'm happy to ramble on about it!