Finally! Just etched a few simple attiny2313-based boards, stuffed the VUSB stack and voila! So now, my homebrew bench power supply is usb controlled.
The whole thing fits in 1500 bytes of code, including vusb stack!
What concerns the client software, I decided to make an experiment, and wrote it 100% in lua, using lualibusb. I managed to create a simple infrastructure, that takes and hides all the usual things, such as vid/pid/manufacturer matching, handling cases when there are several identical devices attached, so that the actual code that drives the hardware can be fitted in about 100 lines of lua code.
Code is right here, on github