Yep!
I'm thinking about it since years with TW, a mod did it with MTW2 -dlv- but not by real number of population by region, just by city level 1 = x soldier available for the clan (2 cities = addition), 2 = x.
It's possible to do it by real number of population too, for the clan or for a region, but in this case we need a real balance to avoid "impossible difficulty" and small clan army may be
I'm speaking about population number (for me), thus even ashigaru are limited by the people present in the region or total regions, to be realistic we can take 40% max of the total population in army (in peace time, 60% for war time)
If somebody wants to move the poll in general discussion mod thread, gameplay or else, he can...