although I prefer object approach of thinking, I don't see a reason why this shouldn't work?
I just tried to create a simple inc file with a function
function test()
{
Game.Msg("Hello World");
}
and in the scene_init.script I've added
#include "scripts/test.inc"
just below the first include
Then I tried
test();
and it worked. So where exactly is your problem.
And please be more specific of what doesn't work. It would be faster to help you. You can always post debug logs to show us where are you stuck. Also you can read great article about bug reporting clicking on the link in Mnemonic's signature.
let me know what's the actual problem.