Wow. That does not sound like fun...

I know that Cypress makes (or made) a line of interesting USB microcontrollers. They have development kits that you should be able to get to talk to any other processor (some bit-banging required). You should be able to rig up a simple SPI interface to let it talk to your HC12. That, of course, is not the most elegant solution. But if it works, why not?

The hardware involved with USB is pretty easy and cheap. To me, the software (at both ends) is the hard part. But, then again, I am a hardware guy.