If its meant for Overworld teleportation
and meant to be a closed system (e.g. admin controlled / not modifiable by 'normal' players) - why not use the command block? You can easily create a nexus in the overworld itself (
I did that in my vanilla ssp world - simply carved out a room down at bedrock level, set up the needed redstone/cmd-blocks and surrounded the whole thing with bedrock again [yes I do know that it does not matter on ssp]) and set "tp" commands for all places needed. Granted, it's not possible to have this system as a player used way of teleportation.. but if all you want is a fix travelling-network, it would be a viable alternative. And quite easily expandable, as the setup for a cmd-block tp is what? A few redstone, a pressure plate and the cmd-block itself.
Though I do not know the load it would cause for a server, I could imagine that it's still way less than a dozen players hopping between several mystcraft-dimensions loading all kinds of worlds at the same time. I
do know that loading a mystcraft age (even an already visited one) takes quite some time (at least several seconds) to load up. TPing with the cmd-block is much faster.
Ah well, just a thought. =)