It is almost impossible to pull off. With so much moving players, at least 400+ rsbot would not be able to tell the combat level of every player in time for the time in between games.
Rsbots.net's Soulwars bot has a join option where you can join the team with the highest player level average.
I think it does this by counting the number of players and their levels for each team then finding the mean value and, obviously, joining the team with the higher value.