Supply lines?

Lately while studying a bit of historical ancient warfare it occured to me that when playing RTW, or more specifically my mod of choice, RT realism, the role of supplies to the armies is poorly or not represented. Once you raise an army, beside the upkeep cost, it seems totally self reliant. you could raise an army and march it to the other side of the world map without a problem if you avoided resistance.
What i mean is, for example, during the first crusade when the westerners were besieging antioch, they had little to no food or water, and therefore it was more a matter of the besieged starving out the besiegers than vice-versa.
Would it be possible to mod this into the game in a way? Perhaps by raising the upkeep of an army the farther away it got from the capital or the nearest friendly faction city? And also by lowering or raising the upkeep cost depending on the features of the province they were in or besieging, i.e: low fertility=higher upkeep or high fertility=lower upkeep, since an army has to forage and live off the land in most cases.
I'm pretty sure this is unmoddable, but perhaps someone else has some ideas how to make this very real aspect of warfare present in RTR.