The left and right OS (or Windows) keys on the HHKB Pro2 are left and right OS keys, not a left OS key and a Menu key.
This explains why the HHKB Pro2 has the option of mapping the left OS key to Fn thus avoiding "claw finger" combinations for arrow keys, etc. -- the amount of keyboards having left and right OS keys is minuscle (Fujitsu Siemens KBPC E is an example) and no software I am aware of depends on this.
However, it leaves us without a convenient Menu key.
Is there a better way of generating Menu key than emulating it using Shift-F10 (standard at least under Windows)? Are programs known that support the Menu key, but not Shift-F10?