I like the idea, but…
I worry that this could hide issues from site staff. If the topic isn’t bumped and staff don’t track the topic, how will they know about the new reply?
Also, adding “exceptions” like this could be confusing. In one case a user bumping the topic with a post like “bump (20 characters)…” would be discouraged, but that could just as easily be “So I said above that I would test this over the weekend…”, which you would want to be bumped.