I would think you could load the laanx page into a frame on the planeshift page and not include all the planeshift stuff to the laanx page, that way the laanx page is clean with nothing more than you need to register -- say, everything under "Create an account!" -- and the planeshift page can still access it.
Actually the registration page need not be more than just a web form linked to the main planeshift page as well as the fragnetics page. You click on the link and a little popup window is displayed. It seems to me you are duplicating the planeshift menu system on the registration page now, at least in the html even though the files are still only in the one place.
I suppose it comes down to design choices something that works now is certainly better than something that might theoretically be better if you can figure out how to do it.