The idea for this would be a hollowed-out ground. The ground would generate with air as the filler, and a combination of dirt and soul sand on top (dirt as the "fixed top", and a combination of mostly dirt, with some paired soul sands blocks); next to the paired soul sands would be a sign, or a small stone wall (in 1.4), or something similar.

Within this land, creepers are not valid spawners -- this means that in the dark, the undead should show up more often.

Yes, this is a horrible pun involving the english language that might not translate well :-).

Bonus points for being able to say that no monsters will spawn near the surface during the day, even in the dark tunnels, but undead will spawn more often at night.

remember that so far EBXL is interested in natural terrain elements only, so "man made" structures are right out for now.

Yes, this specialty biome is not generally in line with something that Extrabiomes would produce...though when we complete this feature it may be an option for users to do this on their own.

Closing

Fair enough, no church. Any chance of a "hollowed ground", air filler blocks underneath a thin layer of ground?

Hmm, I just realized that a hollowed-out ground, combined with Ruins to generate a church in that biome, ...

How would this function for those people that play in peaceful mode? It would just be a patch of ground with soul sand?

Pretty much. There would be hollow-ed ground under the hallowed ground (hence the english humor)

I would recommend Unhallowed be the type of Biome with Churches being surrounded by Hallowed ground. maybe a Hallowed variant of stone which could be used with Silver to make weapons with an innate Smite ability.

If so, that variant stone should be rare.

Hmm ... stone swords with smite ... lower power, interesting.

Ohh ... idea: The hallowed stone is used in the church's construction. The church has some sort of innate anti-undead protection, and when you harvest that hallowed stone to make your smite stone sword, the church loses the protection.

That might be a little hard to do :-)