How to remap 'Context Menu' key on external keyboard?
Bad news. Apple's XML DTD format that Ukelele targets does not support modifiers outside of shift keys, option keys, control keys, and command keys.

As @Lri suggested KeyRemap4MacBook supports mapping the Context Menu Key, which it calls the Application Key. In KeyRemap4MacBook's preferences go to For PC Users -> Change PC Application Key to see all the supported options. Mappings that may be of use for Emacs: Fn Key;  Shift_L Key; Option_L Key; Control_L Key; and, Command_L Key.