Hmmm... What if we will change it a little? @Rory
I mean instead of teleporting to some map foreva, it will just create a status effect on the target and caster afterwards? Like if we follow anime - they are being transported to Shinigamis stomach. We could make it like they are turning into ghosts with all these lights around them and we could approach it on 2 ways : 1.They are visible to others and they can see others( but cannot fight the living ones) - they are just transparent. But! they can still fight each other(since they fight their enemies in Shinigamis stomach). You could make their world look like everlasting night. 2. They are turning gray and are teleported to some Necropolis lol with some different content, they could still fight and what not (limitless options here).
Now! It should be possible to release the soul from this status : http://naruto.wikia.com/wiki/Dead_Demon_Consuming_Seal:_Release .
Maybe you could drop or buy this mask for like 5-8k or something. The ritual would need to be performed by living ones.
Or if we decide to go with the Necropolis idea we could make it more like - player has to go trough some quests in there, then he gets the mask after finishing it and can send it somehow to the land of living to someone so they can perform a ritual.
Also there should be some limits as well to this jutsu, like once per week or something, dunno. Plus we would have to come up with something so alt abusers won't over-use it.
.....Pssssst.....gf needs second sub *coughs*....