Serg,
In a way, you can control this by the amount of C-Bills each team has. The ability for a team to have X amount of C-Bills and then a battle commander would allocate the amount of C-Bills for each pilot would be good. This could limit a given pilot's purchasing power to only one mech. Now I do not think that is possible.
Remember what the MWLL Devs have said:
"Added possiblity to limit buy options for a factory. (not currently implemented)"
In the future there may be an option of having Mech only battle, Aerospace only battles, tank only battles etc. This could be enforced by the MWLL software. While this is contrary to the MWLL design concept of "Combined Arms Combat" as was reperesented in the fantasy world of BattleTech, this type of software feature would add great flexiblity in the way the MWLL mod could be played.
In the end, these additional features many of us are asking for can be done just through a gentleman's agreement or honor system between the two teams. But in the context of using a game like MWLL in a league setting, the ability to enforce a particular game play mode through software "switches" is a much better, less potentially controversial way of proceeding.