There are already several alternate keymaps for the FTV remote in Kodi:
http://kodi.wiki/view/Alternative_keymaps_for_Fire_TV_remote
You will need to either push the keymap to your Fire TV (via ADBfire or similar) or load it from a USB stick in the case of the Shield.
In my case, I am using keymap #3 for the subtitle on / off feature.
Hope this helps.