I'm musing about this.
The road network limits the amount of troops you can have at one place. Each road segment has a maximum amount of maintenance that it can transfer per month:
Imperial Highway: $100M/month (roughly 25 regiments or 2 legions)
Military Road: $50M/month (12 regiments or 1 legion)
Off Road: $15M/month (4 regiments)
Passing through more than 25 miles of desert halves the capacity, since you have to devote a lot of effort to just supplying water to your own caravans.
Ports and harbors would also have a maximum Maintenance that they could send/receive each month, which would generally be fairly high ($20-$50M/month).
I don't think this would change anything in the game's history, and it wouldn't really change much for short term immediate plans.
It would provide an additional natural limit for concentration of troops: it's possible to sustain 4 legions in the same general area, but it takes some work. It would also explain why the Imperials aren't concentrating and wiping you guys out in Menkgu: they can't mass more than 12 regiments (except at Puttayos) and they don't think they can beat you with 12 regiments if they have to do a river assault. Hence the plan to build the road from Gurogeba to Gedupedo, because then they can walk 12 regiments in supply down to attack you.
Supply management would generally be abstracted, but there would be the option of creating prepositioned depots that would let you briefly overcome the normal supply limits.
Or maybe this is too complicated and no one wants to do this so I should stop worrying about it.