I'm getting the same behavior as before with the Trinket when plugged into a computer versus a battery or a USB wall adapter. Basically, it doesn't work. I've decided to use an Arduino Pro MIni instead and power it with an FTDI to USB and a wall adapter.

I've stitched up a prototype pad with five pockets in it. Three are under my behind and there is one under each leg. I've tested it with one and it registers my weight pretty well. I'm going to finish the prototype and figure out how to deal with the wiring.