OK, so sounds like it is a software issue more than anything.
One idea. Why not use one of the "empty" slots in the force pool. In much the same way that you have Heavy Tanks unavailable until such and such a date, your second batch of "bonus" AT units would function the same as the regular ones, but would become eligible and get replaced in a separate queue after a certain date.
Or another idea. Can you not simply set the force pool caps to rise at a certain point in the game (e.g., 3 AT units in 1941 and 6 in 1943 or whatever)? I know the force pool cap for the number of available HQs in the force pool can already be increased with tech advances in Logistics, so this doesn't seem like it would be too hard to do.
Just a couple of thoughts.
I guess it just strikes me as odd that you would have this hard cap on the force pool only to allow it to be exceeded for as long as you want as long as you don't kill off any of the units. I know that as a player, I will try and target those units when I can because I know that they won't be coming back. That seems to reward a gamey strategy.
So yes, I think it's a case of development priorities rather than design priorities.
I think you're right, the best answer is for the force pool to increase on certain dates that match up with the events, regardless of whether the events fire or not. So an extra tank slot becomes available in early 1941 to accommodate the Afrika Corps. Would need some attention to exactly what the limits were before and after (does Germany go up from 6 to 7 tanks in 1941 or from 5 to 6? Does Russia go up from 6 to 8 tanks when the Siberians are due to arrive or from 4 to 6 or in that case is it just 6-6?)
Maybe the devs can consider this for a future patch - though it is creating a whole new system in order to iron out a wrinkle, so I suspect they will not prioritise it.