In theory, I know a way to deal with hell without cheating, and even without obsidian casting.
Due to FPS deaths I've never been able to fully test this out.
As far as I know, clowns are not [WEBIMMUNE], right?
If so, Giant Cave Spiders are the solution. The beautiful thing about GCS is, that you can set up a corridor with traps (cage/weapon, whatever), and use the (untamed) GCS to shoot webs over the traps, using a bait animal.
basic layout to make such a corridor:
###$### #=wall
#/////# $=rope, with bait animal
^^^^^^^ /=retractable bridge
^^^^^^^ ^=traps
+++++++ +=fortifications
d S d d=door, open, pet-impassable
S=GCS
Note, that GCS will not shoot webs at bait animals further away than 4 tiles, hence your trap corridor can only be 2-wide.
The beautiful thing about webbed traps is, that they negate the [TRAP_IMMUNE] flag. When a creature becomes webbed, the trap will spring.
However, your own dwarves will get trapped too, if they walk over a webbed trap. Weavers can disarm webbed traps though, by collecting the webs. FOr this reason, the trap corridor in the above layout is perhaps better only 1 tile wide, instead of 2. Use traffic zone designations to try and keep your dwarves from pathing over webbed traps, when retrieveing caged demons.
The general idea is, to prepare a long corridor with webbed over cage traps (use firproof cages in case some clowns emit heat, to be sure), to catch the initial wave of clowns. Also prepare some 'alternative routes', also with webbed trap corridor, which can be opened and shut, to force spawning clowns down different paths, if you think your military can't handle the steady inflow from the pits.
When you close one route, your dwarves can reload the traps there, and your GCS can web them over again.
This way, not only can you survive breaching hell, you can even sell it's minions to the mountainhomes
(once the bug that caged creatures get released when taken to trade depot gets fixed)
One little cheat might be in order to be able to achieve this feat in a overseeable amount of time.. Mod GCS to be available to buy from the parent civilization.