I think that if a keyboard already has a "hard-wired" Fn key, if the function you want is not already built in, you need to use remapping software (such as Karabiner for Mac OS X) to remap another key to Fn. When you have done this, you can then define just about any function you want for the new Fn key.
BTW, I recently sold my Poker II, but I have kept my HHKB Pro 2. I also have a RF 87u, and I have remapped it using Karabiner software. On the RF, I have remapped Right Control as Fn, and everything else is set up as close to the HHKB Pro 2 layout as possible.