I have this in my Un-Impossible Mission, currently being tested by the OFP in-crowd over on the beta testing board. (You get it only if you take the Hill and the Base and then, in the hope of getting the Easter egg, elect to clear the island of the remaining enemy. Its a good egg, you'll like it.)
My version has a radio trigger:-
Activation: Radio Alpha repeatedly
On activation: newCount = true
Then there is another trigger
Area: whole map
Activation Box: West present repeatedly
Condition: this and newCount
On Activation: hint format ["There are %1 enemy loons remaining.", count thislist]; newCount = false
Every time you make the radio call you get a hint telling you how many enemy are left.
There is only one small problem with this: it gives the wrong answer. Or rather, the total reported sometimes goes up. I'm not entirely sure why this is, but I think at least one cause is vehicles. The vehicle is counted as one but if the crew get out they are obviously counted as two or three or whatever. I suspect there is another problem but I can't for the life of me figure out what it is because this should work.