Currently screen.d/ldm hard launches X based upon options from lts.conf. Meanwhile screen.d/xdmcp does not support many of those options yet after it was rewritten to come out of the stoneage. screen.d/xdmcp and screen.d/ldm should share X prep code. Implementing this might be un-fun because one is entirely shell while the other is entirely C.
This is done, although all options possible from lts.conf are not supported by Fedora yet. We don't handle giving it a xorg.conf location from lts.conf at all yet.