I know nothing about modding, but I wonder if something would be conceptually possible. It doesn't matter if the solution is weird and clunky...

I've read a few times that the distance to capital penalties in RTW/EB are hard-coded and there's nothing you can do to alter them directly. But I was wondering if there's any way they could be tackled indirectly. E.g. could one create buildings that give a gradually increasing bonus dependent on distance to a faction's capital?