Wait.. what?
Actually clicking on demo -> Properties -> Open With -> Custom command 'PATH/TO/LAUNCHER +demo %f'...
%f is full path to opened file..
Aaaaaand you will get error cuz stupid UrT is looking in demos//home/blablabla...
So either you have to change %f to show just filename and symlink it somehow, or maybe some ../ hack
or made new script which do the work :/
aaaaaand if u solve problem up there you can associate the file with your FireRox so you just click on demo and choose open and tramdadada you have automatically launched game and demo...
Just Random Noob: Wow, good job on that +demo thing!!!