Why force people to fly a certain plane?
Because it would be a difficult challenge.
For example, try to get 100k/0d in a 109 K4. Now repeat the same feat in a 109 E4.

Spit XIV - Spit I
F4F-4 - F4U-4
Getting 100/0 in a La-7/P51D/Tempest/262 is probably not all that hard to do if you are not only good, but can fly like a eunuch. I could get it in the 262 without breaking a sweat, yet get routinely pwned by the leet crowd - and also by some much less so.
You could do it this way:
100 points for 100/0 in any plane
250 points for 100/0 in mid war planes
500 points for 100/0 in early war planes
They could even be scaled like the other achievements:
100/0 in any plane- 100/0 in mid war planes- 100/0 in early war planes-
10/0 - 10 points 10/0 - 20 points 10/0 - 25 points
25/0 - 25 points 25/0 - 50 points 25/0 - 75 points
50/0 - 50 points 50/0 - 100 points 50/0 - 150 points
75/0 - 75 points 75/0 - 175 points 75/0 - 250 points
100/0 - 100 points 100/0 - 250 points 100/0 - 500 points