I ported V-USB, the software USB stack from Objective Development for MEGA/TINY AVRs to XMEGA.
It's not tested much yet and still needs some work to make the hardware side fully configurable, but it does enumerate and pass data over HID. I'll keep improving it as I work towards building an XMEGA bootloader around it.
Why not just use an XMEGA with USB? Well, they don't make an E5U yet, and the ASF USB stack won't compile into 4k with GCC. This code is only about 2200 bytes for HID. It is build for the A3U because that's what I had handy. It may be possible to add support for the USB peripheral, but probably not within the same code base as the software stack.