In some other games I've seen the supply rules implemented as tracing routes up the HQ hierarchy. I wouldn't want to change around the whole way things are done, but several people have asked for "supply dumps", and rather than introduce additional units/facilities that have to be moved around or created on the map, I'm wondering if a lot of the unwanted behavior of the current system could be smoothed out with a simple change to HQ behavior and a few small changes to the supply algorithms.
Right now from what I understand, the modeling is based on each unit drawing it's own supply all the way from the capital and they all compete to draw through bottlenecks of limited throughput, in order to try to maintain a 30-day supply in their own current province. What if instead, units used the same algorithms, but instead of drawing all the way from the capital, they would each draw from their own HQ, which in turn would draw from it's HQ, and it would only be the theatre HQ's or other unattached units that would draw all the way from the capital.
Set it up so that each higher level of HQ would attempt to stockpile a greater amount, based on the sum of needs of all its attached units, i.e. a Corps HQ might be stockpiling say a 45day supply for the entire Corps, an Army HQ a 60-day supply for the Army, etc.
This would mean that all the combat units would be only needing to trace a much shorter path to a nearby HQ, and each path is only possibly competing with paths from a few other nearby units that are drawing on local HQ supplies. I think you would see far fewer of the inexplicable out-of-supply situations, and with your major trunks branching out through higher HQ's that aren't moving around as much, the system should be much more stable, and problems should be much easier to visualize, understand, and fix. For instance:
You moved your Army Group HQ several provinces off into the low-infrastructure sticks and notice that it's supply reserves are dropping. Well before it runs out and starts causing problems for your actual combat units, you fix the problem by moving the HQ back a couple provinces where you had no problems before. You realize you need to build up a higher-infrastructure corridor if you want to continue moving a whole army group off in that direction...
That is so logical that it scares me that it was not in the game from the start...