For server operators worried about expiring mystcraft ages. Simply do find command and purge files > 30 days, just RM -rdf that dimension's map files. It really is that simple. Worse case someone who hasn't been on in 30 days spawns were he last was and regens those chunks. If you haven't showed up for 20 days, we just assume "your dimension had imploded." We don't want players living in Mystcraft worlds (forcing them to stay pretty much in overworld only) and use mystcraft for exploration and resource exploitation. We are starting work on a simple mod that only allows players to stay in non-overworld locations (including the nether) for about an hour. After that is starts deducting levels from the player every 5 minutes. If they don't get out before hitting level 1, they get ported back to the overworld default spawn. While in the overworld map they recharge that buffer 2:1. So they have to hang out in the overworld for about 2 hours to refill that. That is how we are planning on balancing exploiting mystcraft dimensions (well any dimension mod really) for resource gathering. We also disabled chunkloaders from being placed in non-overworld dimensions. We wanted to limit the whole "free lava energy" bit that makes the game way way too easy. Now we have to hoof it out to the nether, drop like 6-12 pumps, and top off our lava tanks back home. Advantage on a PVP server is, you better take all your gear back with you because there is a good chance it won't be there when you come back. Without limiting infinite lava, there really wasn't any point in any other powersystems and no one was building anything other then magma engines and geothermal generator farms. Infinite lava = boring gameplay. If you are worried again simple steps:
Disable chunkloaders in non-overworld locations is one of the easier balancing tools you can do. It doesn't matter if you have infinite X in a dimension if you can't drop a chunkloader. It's not a perfect solution, but it's better then nothing.
Culling old dimensions: just a simple "find . -type f -mtime +30 -delete" will nuke any files older then 30 days. Just run that in the region directory in any dimensions you don't want to save. You can write a simple script that compiles a list of all the DIM-X locations, make a whitelist of DIM-Xs not to delete and run a "find ./world/DIM-X/region -type f -mtime +30 -delete"