A simple framework for linking scenarios in a campaign like setting would be as below. Note that all the numbers are adjustable depending on the campaign, and that I pretty much pulled them out of thin air but the feel like a good starting point as long as the pool size is at least 3-4x the typical scenario size (give than very few units tend to survive most GEV map scenarios).
Before the campaign begins, each side has a base pool X AU of armor and Y squads of INF available (exact units don't matter, it's a generic pool). The scenario calls for N AU and M squads of INF, which come out of that pool. After the scenario is over, all surviving units (including escapees towards friendly lines per the scenario) go back into the pool either as generic AU/squads (simplest) or as the unit types they were (takes a little record keeping). This is your base pool available for the next scenario.
After each scenario, you might get reinforcements into your pool depending on how you've been doing. Roll 1 die, adding 1 for each level of victory above draw or subtracting 1 for each level of defeat below draw from the previous scenario. Use the following table to see what your pool gets:
- 0 or less: LOSE 10% of your starting pool (they need units elsewhere more than you do!)
- 1-3: You get no reinforcements at this time.
- 4-5: Gain 10% of your starting pool.
- 6 or more: Gain 20% of your starting pool.
If you ever find yourself in a position where you don't have enough in your pool to fully populate a scenario, tough luck (you can trade 1 AU for 3 INF or vice versa if your pool has enough, but only to fill missing units or as the scenario dictates). Additionally, your total pool can never go above 125% of your base pool, or 20% more than the total AU+INF required by the biggest scenario in the campaign, whichever is bigger.
I don't think that there needs to be an exact formula to cover every single situation, just an agreed upon set of numbers before the campaign begins. You could also add in extras like random events that might give extra reinforcements (or take them away), along with checks to see if surviving units break down (physically or mentally) after battles, but that might be more complicated than it's worth.
This system doesn't take into account any sort of unit experience, as that requires a lot more record keeping than I think most Ogre players want, but you could easily add something like that if you wanted to (assuming you tracked actual units, and not just generic AU). I'll leave the ideas about that to someone else :)