It would certainly be interesting to bet able to set a bunch of priorities. Not just bad on template, but how much experience is as you mention, as well as whether to focus on many full strength divisions or few low strength divisions, theater, etc. It doesn't seem particularly feasible, as it's a little too predictive (in order to reinforce one unit, you need to know the status of another unit), but interesting.
It seems like it'd be a fairly easy system to set up? I mean, just as a faux mockup - assume we have three priority classes of troops, High, Medium, and Low.
Get the number of Reinforcements available for the time increment, we'll go with a week.
Take two thirds of it, and earmark that for High priority troops.
Take two thirds of the remainder, earmark it for Medium.
Mark the rest for low.
For each class, randomly assign reinforcements point-by-point to units in need. If there is a remainder after all needs are satisfied, send it to the highest priority bucket that still has troops in need of reinforcing.
This wouldn't be a perfect system, obviously - for one thing, if you have 300 reinforcement need in High, and 50 need in Medium, and 300 men coming in, you'll get ALL the Medium filled (1/3 of 300 = 100, 2/3 of that = 66 > 50) despite not filling all the High, and that's not what we want. But you can mess with the basic idea until you get a thing you like.
For example, on a "focus on keeping divisions combat effective" priority structure, you could make the gradations instead be different levels of strength in the troop - divisions just too small to be effective getting most of it, divisions that are flagging but still in the fight getting most of the rest, and divisions that are either near paper strength or utterly demolished getting only a trickle of troops. Bottom line is that you really shouldn't need any sort of n-body problem style interunit referencing, just taking a priority value from / cooking one up based on the characteristics of each division in a vacuum.
What would be interesting to me would be the option to have reinforcement restricted to units under specific commands - so I could let my troops on the front fight and gain experience undiluted by new blood, while building new formations in the rear. When a front unit gets too small to be effective, send it rear to form the nucleus of a new unit.
