As I understand it, it is imposible to survive opening a passage to hell. This sugestion makes it posible but very hard and also gives the adventure mode player something to do as a quest or by his own initiative.
DF mod
Instead of having a continuous flow of demons, they would come in waves, big and strong waves. The thing is to give the player time to cap the hole after he defeats a wave.
The hole can be capped with adamantine walls.
Before the wall, an adamantine hatch or door can be installed, it will eventually be destroyed by demons, but it will slow them down.
If the things above seem to easy, some of all of the following can be done:
Some time after opening hell, a big explosion creates new tunnels that reach the closest cave, this would counter the tactic of making a long corridor that can be capped far away from hell entrance.
Having a simultaneus night creature/undead/etc. invasion above (or similar). The argument is that demons call evil to them.
Hell can only be closed at entrance, the adamantine wall you make should only touch other adamantine walls (natural or constructed). The argumet would be that adamantine is holy or simply too durable for demons to destroy. After all, Hell is enclosed in adamantine right?
Wave strenght and numbers could be somewhat randomized. The argument is that not all the demons in hell are just waiting at the place where you are mining.
Your depressed dwarfs turn hostile. They may also become monsters somehow (Doom style)
Your dead reanimate and become hostile (some may be friendly too). Other dead creatures in your refuse pile reanimate too.
Plagues, husking clouds, etc may be generated from the hole, this effects may have a short duration and come with every wave.
You can make up other mechanisms to make it posible but hard. I think that if all of them are implemented it would be imposible to stop the demons, maybe it could be done if the hardening mechanisms are randomized (you may get only the demon waves, have a few hardening mechanisms or all of them).
Adventure mode.
In Dwarf talk several scenarios have been discussed about a hell breaking scenario. In general the idea is for it to be a really big trouble for the world with demon numbers increasing in the surface. The posibilities range from a demmon army invading all the land as a united force, to demons spliting into groups and fighting among themselves.
I sugest 2 more posibilities:
Portal scenario
The first demons to enter conquest the fortress and not wanting to share power with the demons still in hell, close the entrance with a magical portal that lets only the demons the choose get in. In this scenario demon population above ground increases slowly and fewer powerfull demons are spawned.
Portal can be created in other ways like taking the adamantine sword or when the stars are right a portal can be created in the surface (maybe those portals can have a limited duration).
The adventurer can use magical items or eventually maybe even his own magic to close the portal. But if he does not destroy their presence in the fortress, other demons may reopen the portal.
Becouse of how dire the situation is, other powers may be involved like gods, angels (if implemented), etc. All races are more likelly to get united (not goblins) or at least stop their wars. If dwarfs open hell they may get attacked or denied trade.
Direct passage without portal
A worst case scenario, either the demmons getting in at first are not that selfish or they belive themselves powerfull enough to boss wichever demons arrive afterwards. Demon population in the world increase faster and lots of powerfull demons enter the world.
An adamantine door or hatch was to me taken to cap the hole. Becouse of the weight you have to take companions.
Becouse of how dire the situation is, other powers may be involved like gods, angels (if implemented), etc. All races are more likelly to get united (not goblins) or at least stop their wars. If dwarfs open hell they may get attacked or denied trade. All of this is more likelly in this scenario.
A direct entrance would make it more likelly to have more ill effects on the land over the hole and in the fortress, evil type biomes could spread fast, the passages of the fortress could become contaminated, husking clouds could be generated, etc.
I realize not all of this may be posible now, and that some features may need to be implemented before.
What do you think?