Today I started playing around and putting the core components together; I am glad to say that I think this may work!

The powerboost seemed to kick out enough 5v juice for both the Pi and screen to run from my 3.7v batteries :-)

The screen's backlight requires 12v so i may have to add a step up coverter to get that running, and i may also add a 5v fan as the powerboost was running a bit warm.

I am also going to order the powerboost 1000C and swap out the 500c; it could do with the extra power out!

Still a lot to do: adding a portable hard drive and soldering the buttons up then working out how to fit everyhing in!