I did this so long ago so I might be wrong. I do not remember it being too difficult. There was a keyboard.xml file somewhere in the userdata folder of xbmc. In there you add a line, that follows the pattern of the other keymappings. It would probably be something like "<Left>contextmenu</Left>", where you change "Left" with the button you want.
I know this is a bit vague, but I guess its a start in the right direction. I will let you know that it is possible. Also google's your uncle.