You know Aussie_Lurker, there may be a way of adding a crude form of 'supplies' to the existing
game by simply making a few simple changes with the current PTW editor.
(I love pushing the editor to it's limits and make it do things that it was never designed for.
Am I the only person in the world who thinks that it is a bit 'odd' that Radar Towers can make INFANTRY & TANKS fight 50% better in both attack and defensive situations?
Aircraft, yes. But other land units?
So here's the cunning plan:-
Instead of units fighting worse when they are 'out-of-supply', why not have them fighting better when they are IN SUPPLY!
Change the name and graphic of Radar Towers to Supply Depots!
Now, if you are within two tiles of a friendly Supply Depot, then your units will fight 50% better, as they are IN SUPPLY.
And if Workers use a new Worker Supply Wagon animation, you will have columns of these wagons following your stacks as they advance
..wagons that need to be continuously built because they are used up whenever a Supply Depot is constructed.
Using these wagons as workers will also give the visual impression of supply columns all over the place, especially when invading and new roads need to be built by these worker/wagons as the army advances.
They can even be captured from each other if attacked! They are after all just workers with a new look.
I believe that mrtn is currently working on a new "Supply Depot" graphic (I suggested something like a couple of small triangular grey tents, one larger square brown tent, and a couple of piles of barrels & boxes....plus a flagpole would look nice.)
I myself am working on the "Worker Supply Wagon" animation, which has an 18th century 'tricorne' hat in this preview....
....but I also plan to make a more modern version, with a lorry or truck in the background.
Although ancient & medieval armies could surive for quite a while by simply foraging, this was certainly not the case once Gunpowder was discovered. So I suggest that these "Supply Depots" become available at the same time as Musketmen, and thus represent all the power/ammunition/shoes/new uniforms/replacement firearms/etc that more 'modern' units required.
It's not quite as in depth as your suggestion.....but with just a few graphic & name changes, we can implement it NOW!