I have set up NetBSD with XFCE4 and one of the problems I recall was the choice
of Xorg. You can use the sets that come with the install CD or install a modular Xorg. Google for 'modular xorg NetBSD wiki' for more on this.
Your post indicates you will pull LyX from pkgsrc so I would install the Modular
Xorg from the same version of pkgsrc.
I'm running Arch linux and recall that there are alot of on wikis on how to disable KMS. Xorg 1.8 just went into testing in Arch Linux.
Another thought. I'm not familiar with LyX but Slackware has version 1.6.3 in the SlackBuilds page. I have had some sucess editing the build scripts to newer versions. This may be one way to get an new version of LyX with an older Xorg.
http://www.slackbuilds.org/repository/13.0/office/