If you have 0 map participation when the elite boss is killed you are ineligible to get one of the infrequent reward chests. If you merely have low contribution, then you are able to get a brand new version of the torso which buy Maplestory 2 Mesos only a small subset of the prizes. Probably something unlike any CSS without a epic pot scrolls IDK. If you have high contribution you get the routine, purple rare reward chests. And the more you have, the further chests you get.
To make it work, it would need to be time gated. What I mean is that you would need to maintain the map actively killing mobs, for many minutes before you begin earning contribution. They shouldn't be getting anywhere near precisely the same contribution as the players who actually put in the MS2 Mesos job to spawn the boss. This keeps people from moving in, killing several hundred/thousand of mobs to get high participation and only camping at the map afk on a stage protected from the elite boss and gaining a great deal of benefits for doing nothing.