The clowns don't simply destroy the floor grates?
I have been playing around with breaching the HFS and systematically "dealing" with the initial onslaught, and encasing them all in obsidian seems a much more elegant solution than pulling them all into a room and dropping a giant pillar of earth on their heads (I actually did this, but it locked my game up for some reason >.<).
I've been working on a few "hidden" creatures for my mod that wander around down there. Basically, hell, once breached, behaves just like the other cavern levels, with wildlife "wandering" on through the edges - only the "wildlife" happens to be the clowns. Adding different "varieties" of wildlife down there in the raws seems to work just fine, in my experience, showing up in the world sites and pops list as "unnumbered." However, they don't seem to ever be friendly with the other denizens of hell, which usually means that anything wandering onto the map down there will get curbstomped in a hurry unless it's obscenely powerful. I haven't played around with adding [AT_PEACE_WITH_WILDLIFE] - I'm curious as to whether or not this will keep them from attacking the clowns.
Ultimately, I want to turn hell into a sort of "end game" cavern level - a final challenge to a fortress that has overcome all other challenges, with all the rewards associated with that. It's been difficult to reliably test these concepts without a method for actually getting down there and staying alive long enough to observe wildlife patterns, but this checkerboard system might be just what I need for it.
Also, another curious thought. I imagine that adding plant life to hell will not result in anything happening on its own down there because of lack of suitable soil, but I wonder if adding a "hell" plant will make it spontaneously start to grow if you breach hell and then create a mud patch. If so, this would be a great idea for an easter egg - Kill demons, breach hell, get rewarded by (otherwise unavailable) demonic grapes. Again, just me thinking out loud here, but hopefully this little device will give me a way to test out some of these ideas.