This is not currently possible. The game does not track that information.
Could something similar be added to the same database that tracks each individual unit though? Actually, that seems like it would be a lot of work.
Could the units CURRENT training, have a modified (diminishing returns style?) effect? Where Very High and High Training decreases more rapidly, but lower training (say under 50) slows down and takes a much greater and prolonged time? With the idea being that once under 50% training, there's not a lot to lose, and once you hit like 35-40% . . .well, even the most fresh recruits can do that, so they can't really drop too much lower (and if they did, it wouldn't be too noticiable).
Most units I see - even low end ones usually have 40-50 training (I think most realms destroy ones that are really low) - so it's not unreasonable to put some lower end cap on it. This also doesn't completely ruin Militia, and just effects the more well-trained higher end units that most nobles like to carry around.
