I have been working on something as well. I think that you have to decide if you are going to use CM to resolve all combat that takes place in the campaign, or will some combat be resolved using some form of abstraction or other rules system. Using CM sounds all well and fine but I think that it limits the size of the campaigns to a handfull of units on each side. It will also make for little more unrealistic OOB as to make the CM games enjoyable one will have to mess around with the unit's makeup. Units will all have to be able to compete within CM to have any use at all. How many people would want to fight CM battles that are 100% infantry vs infantry, or a lopsided battle of tanks vs infantry. I think that the interest in the game will wain if this is the case over and over again. This will probably slow the pace of the game down, and that would be deadly to the campaign.
Now if you go with an abstracted "other rules" type system, you complicate things up somewhat, but the advantage is that the campaign will procede quicker because uninteresting, lopsided, or excess battles can quickly be simulated. Campaigns can be larger in scope and you might get a better, more realistic feel to the campaign. Problems are the book-keeping, and the conversion of units from CM to the "other rules" system. These problems are huge however, and probably the demise of many good campaigns.
I have played around with a spreadsheet that calculates the percentage to be used based on the CM casualty stats from a battle.Problem is I am not a spreadsheet design guy, so trial and error rules the day, mostly it is error. The trick to keeping things straight with tracking casualties in CM is to understand how the forces are organized and listed inside CM. Then all you have to do is duplicate this within the spreadsheet and it becomes a matter of "down arrow"/number of OK men/"down arrow"/... when you get to the end the percentage for the next battle is calculated. If at any time you drop below 60% on your base unit, just eliminate a platoon or two and addin the men here and there how you want as you are only looking for the final percentage numbers anyhow. I think that absolute accuracy as to wheather a machine gun is lost or not is rather overkill, just go on the base unit and percentage and let CM do what it has to in order to make it work.
The only simple way that I see to marry the two systems is a points type systme. Each unit is recorded as x,000 points, these will be spent within CM as they are. Losses are based on the outcome of the battle, and not what actual damage was done. So a total victory might represent a 80% loss to the losing side and a 20% loss to the winning side and so on down to a draw which would be a 50% loss to both sides. Then the unit would be recorded as the new x,000 points unit after applying the losses. HQ's would have x,000 points to reinforce the units as they see fit, but they have a limited number of points to do so. Standard rarity is used and and based on the parent unit type ect. When it comes to a battle that for whatever raeson that you have for not wanting to play it out in CM, you develope an odds based system that just compares the total points of one side to the total points of the other side that will meet in combat. It should not matter the makeup of either side as you are compare same value of the two forces involved, that being the total point for each side. You will come up with a battle ratio such as 2:1, 3:4 or whatever and calculate the battle result with a modified dice roll based on terrain ect. and you will come up with a CM equivalent Total loss, minor victory ect. for one side or the other and apply the same losses as you would with a battle foughth using CM. I would make the losses rather high as it was not uncommon for a company to take 60-70% losses in a given battle.
Just a thought.