I'm guessing this is either simple or impossible. As part of a little mod I have a cell which is, in theory, buried under Sadrith Mora. Both for realism and convenience, I'd like Almsivi & Divine Intervention to work there as they would in the city above. However, the fact that the only access to this cell is via teleportation seems to break the Intervention spells.

So, is there any way I can get it to behave as if it's an ordinary building in Sadrith Mora?

I don't understand your problem. :-/
1.
Q: You have a cell in which you have no doors and you want to be able to almsivi/divine intervention there.
A: Put an Almsivi/Divine intervention mark there names are Templemarker, Divinemarker. They are both statics.
2.
Q: You have a cell in which you disabled teleportation but still want to be able to divine/almsivi out there.
A: You might be able to do this with a clever script (detect if the player is casting divine/almsivi and if yes enable teleportation otherwise disable).

No, teleportation is enabled in the cell. You can cast the Intervention spells, but all that does is give you the animation.

I'm guessing I could fix this by putting in a door to the outside, but placing that door somewhere the player can't actually get to it. Still, that strikes me as an unclean solution, hence this post.

Edit: Indeed, slapping in a door to some random point in Sadrith Mora makes Intervention work properly.