I went with the simplest solution given available parts and created thoughtfix's version of the power injector.

I used the severed female ends of two USB A extension cables which hang about 2 inches out of a black project box. The project box came with a PCB board which made for easy soldering/packing. The voltage regulator showed 5.01v out on my multimeter and all's well.

My logitech keyboard only worked in xterm and the enter key brought up the onscreen keyboard. It didn't work in any other apps. I think it's the keyboard, it's wireless so kind of flakey in the way it works. My 40GB mp3 player mounted fine, though it uses fat16 so I'm stuck with short filenames like "Simps014.93j" instead of "Simpsons - Season 15". The USB key works fine when mounted.

I've never seen this before but I'll guess it's normal; the sdx# devices don't exist until something is plugged in, and depending on what I plug in, it's different. My mp3 player was sda, my USB key was sdb1. Maybe this is just the order I plugged things in to test? I'll check again later.

Much thanks to everyone that's ever posted here on this topic, and especially to thoughtfix who had a blog clear enough to explain this to someone who slept through half of his highschool electronics classes and never made anything more than a stun gun out of a disposeable camera.