Long story short, I painted my BOC screen with RS-MaxxMudd-LL. I also made panels of C&S Ultra from Hometheatershack, the Sherwin Williams ProClassic recommended in the Projector Central article, and unpainted BOC to compare them directly.
I guess I shouldn't really be surprised by the results. In order from least light to most light, they go: BOC, C&S-U, RS-MM-LL, SW. And by "light" I mean both brighter whites and lighter blacks. The closest of the four are RS-MM-LL and SW, with the Sherwin Williams edging it out a bit in lightness. The SW is actually not quite 100% dry yet so I'll check it again in a few days.
Given the fact that presumably someone wanting a screen of these shades is probably after brightness, I give the nod for now to Sherwin Williams. I should point out that though none of these had both brighter whites AND darker blacks than any of the others, for me there is more apparent difference between a brighter white and a dimmer white than between one shade of black and a slightly lighter one. Not only that, but the brighter whites made that panel's corresponding blacks seem darker in comparison. In other words, as far as perception goes, I saw far less difference between the blacks of the four than between the whites, so for me at least, going up the scale in lightness actually improved perceived contrast. On many scenes, for example a hallway of the Nostromo in Alien, a scene from the Road, or heavily detailed scenes like the royal court in the sacrificial scene in Apocalypto, I had to really look hard to see where one panel ended and another began; while on brighter scenes, like snowboarding in Art of Flight, the difference was immediately and significantly apparent.
Color was noticeably different on all four on a blank white screen, but in every movie scene I put up, any color differences were for all practical purposes undetectable to my eye. The guys at HTS seem to be the only ones who bother to actually test/measure this kind of thing, and I believe C&S and BOC are both very neutral with SW and then RS-MM-LL coming in behind.
Sherwin Williams is off the shelf, costs $20 a quart, and supposedly rolls on like a dream if you don't want to bother spraying.
The RS-MM-LL costs about $80 at a minimum, although this gives you a much greater quantity. It is not an open the can and go solution, due to requiring 4 ingredients and more particular quantities.
C&S Ultra is around $32 for a quart or something like $52 iirc for two quarts, assuming you get the BASICS silver with a 40-50% off coupon (that goes for my stated cost of RS-MM-LL as well). Only two ingredients and simple to measure though, just equal amounts of each of the two ingredients. Note: I might be mis-remembering and the BASICS might have been $2.50 with the coupon rather than $5. If so, it would be $22 for a quart and $32 for two quarts. I can't remember if the regular Michael's price is $10 or $5 per 4oz tube.
BOC is almost always on sale at Joann's and there are almost always half off coupons as well, so usually work out to $7 a yard or less (54" high).
BOC requires a frame which is around $20 of material at Home Depot. The total costs to build my frame for a 126" x 54" 2.35 screen and get the BOC were around $40.
The paint solutions can be applied directly to a good wall or another material. If your wall is in rough shape and you want to paint on the ideal surface, a sheet of Sintra, that will cost you; a 5x10 sheet to almost-but-not-quite match the dimensions of my BOC screen would run you $95-100. I went with spraying and personally found it an enormous chore. The time and effort to dropcloth off the area so the paint dust doesn't get all over the place, the cleaning of the gun between every coat, and the (for me) difficulty of getting a good smooth result had me wishing I had rolled or just stuck with BOC, to be honest. Not to mention $80+ for the sprayer, mask, dropcloths, and other supplies. I should add that I'm not handy at all, though.
Personally I'm not sure why BOC gets kind of short shrift as a "passable starter screen". It seems to me that it has a niche just like any other solution regardless of cost, that it fits right into the spectrum, having darker blacks than the brightest white and white+metallic screens, but better whites than the grey screens. Add in the extremely low cost and the ease of putting a screen of pretty much any size together, and I don't see why seemingly everybody rushes to either a grey screen or a paint solution. I would imagine there'd be many projectors, screen sizes, etc for which BOC would be the sweet spot of white/black performance, just like any other solution.
I'm sticking with the RS-MM-LL because it's brighter than my original BOC -- important on a 10.5 ft wide 2.35 screen with a Mits HC4000 -- and like I said, the hassle and expense of painting is enough that I don't want to repaint it. When I move next year, if I have a good wall available and the same projector, I'll most likely paint the wall with the Sherwin-Williams. My new screen is significantly brighter than my original BOC in bright scenes, as I said, but honestly I wouldn't have known it if I didn't have a BOC panel to compare it directly to.
Thanks to MississippiMan and Harpmaker at HTS for all the help and suggestions.