AFAIK, it doesn't have one (happy to be proved wrong) but what programs like shortcutd is listen on D-Bus' System Bus where an event is emitted upon the pressing of the key. Short of having access to the code, you can't make Mobile Maps listen to it. You could nicely ask hopbeat/hqh, Shortcutd's developers, if they'll add an option to shortcut allowing it to be mapped to another key other than backspace.