Kal himself here would be pretty open about the idea and probably share it on the network (as SUPERMAN and not Clark), suggesting a meeting for people who could be interested to try to approach the attack this peculiar way so discussion could be pretty upfront if people are willing to meet?

Also he isn't stupid, he knows just punching it to see what happens wouldn't probably help (and he might or not might have tried already and might or might have not gotten a new skin color as a result who knows), so if you need your character to be discouraged from doing that he can help as well. This is more for an actual planned attack.