Went with
etrailer.com - good selection, free shipping (over $150), and even though I got a $300 order in to them at noon on Monday it will be here from MO today. Simplest and best experience I've had so far, with Frank Bear from Vintage Technologies being a close second.
I did decide to go with the lights I'd been asking about, even though I know they're not likely as bright as the "certified" backup lights, primarily because they aren't - as I said in the original post, I'm looking for something a little dimmer to not annoy folks in neighboring sites if I arrive after dark.
The temporary "get it on the road" expedient is a set of standard LED boat trailer lights mounted on the fender brackets, with the backup lights being mounted under the trailer behind the wheels. Eventually I'll fab a set of light nacelles/boxes that will be adhered to the lower rear curve of the trailer and look something like Slowcowboy's setup, but using four oval lamps per side - red tail/brake, amber turn, and two intensities of white (with a switch to select).
Whitney