I've been looking into this. Been using Unifying devices for 3 years or so, multiple devices, both keyboards and mice. In my experience, it's always performed reliably: no noticeable lag, range has never been an issue, and battery life is outstanding.
The most straightforward way to convert a keyboard to Unifying is to get a cheap Unifying donor keyboard, tear it apart and put its controller on the keyboard you want to use. Gon does it:
You can even change the layout with a little more work, once you have mapped both matrixes. But you can't reprogram it or otherwise change the layout easily afterwards.
If we could get a reprogrammable controller like TMK or Soarer's to output to a Unifying keyboard controller, it would be perfect. I wonder if a Teensy++ 2 would have enough I/O pins for that.