Thank you very much burke and cposada,
Especially for the straightforward answer:
<!--Template XBMC keyboard file--> <keymap> <fullscreenvideo> <keyboard> <c>RunScript(script.xbmc.subtitles)</c> </keyboard> </fullscreenvideo> </keymap>
It turns out I was WAY WAY overthinking it when it was actually this simple. I wasn't seeing the forest for the trees!
"RunScript(script.xbmc.subtitles)" was mentioned in a few places in xbmc wiki's etc. but nothing direct as is put THIS, HERE, like THIS....
The next version of XB