Not mandatory to use CMP maps. I'm up for the idea of making these events if official maps are used. All the mapping needed would be to add rally point's mechanic, which is 1 line of code and can be automated. That was excatly the mistake made in the past, CTF, TeamCup, etc. New gamemodes which need lots of work and mapping preparation, usually in CMP maps which also need work...
This can be an opportunity to play push maps the right way. I personally love that kind of gameplay style but it isn't played as it should in public servers. Usually only 1-2 squads per team focus on teamplay.
Also this can be used as a test platform for new mechanics that could be later introduced to campaigns.