I first sawed down through the washer, so that I wouldn't have to undercut the washer to get the head of the bolt off cleanly. I was also working partially on the theory that the head of the bolt and the washer, instead of being two pieces, were actually one. The photo below shows the status of things at this point:
Now, with a good vantage point to cut the bolt, I laid the bike down on its side and began cutting. I was able to saw through the bolt cleanly, flush with the top of the stem (I stupidly didn't get a photo of this). Turns out that the bolt and washer were indeed two pieces, but had partially fused with rust and an extreme case of over-tightening at some point. I then stood the bike back up (resting on the fork and bottom bracket now) and set about trying to tap the bolt down to knock the wedge loose.
Unfortunately, with the sawn bolt now flush with the top of the stem, it was quite difficult to hit the bolt hard enough to dislodge the wedge, and my efforts were not successful. I had already purchased a replacement stem and handlebars (a one-piece deal off a 1960s J.C. Higgins--very close in shape and spread to the original, but the bars not adjustable) on the assumption that I might have to do something drastic, like cut through the stem, in order to get the wedge knocked loose. So, I gritted my teeth and set about cutting through the stem to get better access to the bolt.
I again laid the bike down, resting on a couple of paving bricks padded with a towel, and started sawing through the stem near the top. I cut around the stem, rather than through the bolt in order to preserve as much of the bolt as possible to get a grip on. Here's the result:
Once I stood the whole works back up, I was able to grip the bolt with pliers and turn it loose, which it did maddeningly easily. So, the bolt wasn't rusted to the wedge after all! The problem turned out to be that the wedge was a bit too wedged in the stem tube--so much so, in fact, that just knocking the bolt (and quite hard) wasn't about to dislodge it.
With the bolt removed from the wedge, I was able to get a rather long and hearty screwdriver down in there and after a good deal of pounding with the hammer, managed to finally knock the wedge out. After that, I was able to separate the stem and fork easily, and I even managed to catch the loose headset bearings quite neatly as I pulled the fork from the head tube, allowing me to forego the swearing, muttering, hands-and-knees search for errant bearings.
All told, the only victim was one original, but very rusty, stem. I must say, I really hated to destroy the stem, but I'm not too broken-up about it, since I have the new stem already, and I can probably find an even better replacement at some point down the road. I might have puzzled through a better way to accomplish this whole operation, but I'm well enough pleased with how it turned out.
So, final verdict on a hacksaw as a bicycle tool: quite effective, if a bit drastic. Gives new meaning (or, perhaps, original meaning) to the phrase "bike hacks."
More on the cleaning progress soon, including a stow-away hiding in the frame.