Acidarmor's idea is interesting, but creating a system like that could be a project in itself. However, I disagree with some of the things in his post.
An example of catching up could be a Scarf Flygon outspeeding and U-Turning onto a weakened sweeper, thus KOing the sweeper AND getting another pokemon in.
This may or may not actually gain momentum. On the one hand, you KOed a pokemon, which is obviously getting you closer to your ultimate goal of defeating the other team. On the other hand, your opponent gets to select a pokemon to freely switch into whatever pokemon you brought in. It isn't really an advantage to bring in something after a U-turn that KOed something, because if that said pokemon was death fodder, you have just lost
Actually, I was thinking about looking at warstories and the momentum in them as well. The biggest part of a good warstory is the thought process behind a good battle, specifically prediction.
CAP 1 should make predicting the opponent easier. This can include Tricking choice items on to the opponent, statusing them, Encoring them, U-turning...
Actually, an interesting aspect of the CAP could be an easily exploitable weakness. For example, Lucario gets setup opportunities often from the opponent using a Choice-locked Pursuit to kill a ghost. Lucario then sets up on the weak attack, forcing the opponent to switch, gaining you momentum. (If we are going to have a Momentum-counting system for battles, I still think "tempo" should be our unit of measurement :P)
Therefore, a single glaring typing weakness could actually help this CAP gain momentum for its team.
This pokemon could, instead of only being able to gain momentum for itself, be able to support the entire team by gaining momentum.
Momentum, after all, brings in the concepts of offensive and defensive synergy under what can be considered the "imperviousness" part of making the opponent switch through defensive pressure.
tl;dr Please don't count this as poll jumping; I'm merely stating that pokemon with weaknesses can exploit those weaknesses to its team's advantage in order to gain momentum.