This is not rocket science, but just in case it's useful for anyone else...
You can have an HHKB homerow cursor keys imitation, with autohotkey; I'm using the right shift key as the fn key (I don't use it much; you can map right windows, or any other) and vim movement keys.
Just save this in a text file, with ahk extension, and double-click on it to execute:
; happy hacking imitation, vim style hjkl
~RShift & h::Send {left}
~RShift & l::Send {right}
~RShift & k::Send {up}
~RShift & j::Send {down}
PS: I don't have a HHKB, I'm just guessing how cursor keys work on it...