...C) Anybody have success in crafting a classy tensioner out of, say, old SIS derailleurs?

not a derailleur solution, but a chain tension one sans dropout nonetheless. i think i saw this on chicagofreakbike.org, but i can't find it now. i have used it in my own tallbike application, so i know it works. perhaps a little clunkier than a derailleur fake out, tho. it is pretty elegant, and it looks freakin sweet in action, and people don't believe that it works without falling apart.

just get a loose sprocket or chainwheel that's larger than the cog, and that filles out the chain somewhere between the cog and the chainwheel. the chain would then not go in a straight line from bottom of CW to bottom of FW, and vice versa, but from bottom of CW to bottom of floating chainwheel, to bottom of freewheel; from top of FW to top of floating chainwheel, to top of chainwheel.  hard to put into words, but if you can figure it out, it works awesomely. at first, it seems like it wouldn't work, or it would require a specific number of teeth in order not to walk to the front or the rear, but the truth is that the chain is moving forward (at the top of the loop) just as fast/many links as it is moving backwards (at the bottom of the loop.)

it doesn't take some magic gear ratio figuring, just some "try to find one that fits" until it works. i'm guessing most people on this list, and especially reading this thread will have access to enough different sized chainrings/large cogs to make this work. or at least try it.