This feels more like a feature that the Lemmy server should implemented. At best the client can provide the information, but I think having Summit enforce post limits would be a terrible idea.
As for showing some sort of infographic for post frequency, I can't immediately think of any way to implement this without it feeling a bit naggy/annoying to the end user. With Summit's focus on configurability, I'm sure users will want to be able to turn such a feature off and if all the heavy posters simply turn it off then I don't feel like it would be worth implementing.
I'm going to veto this idea for now unless someone can come up with a better way to implement this.