How to make an USB keyboard work with Bluetooth instead?
You can use Handheld Scientific's adapter which is battery powered.

Here is a build log for a Model M.
I don't know if HHKB has enough free space, but one always can hack something up.

You can also check out adafruit's ez-key, I guess it'll be fairly easy to use it, won't take up much space in the keyboard either.