[AT&T LG G3] wseverin:
It's not that difficult...

Update:

Have built and posted CM13 ROM for d850 with working GPS. See this thread.

It's not terribly difficult to build your own "unofficial" CM13 ROM image for the d850. I did it this morning (12/19). I was surprised that most everything works, more or less, except for GPS which is still dead as a doornail.

The only difficult part of building CM yourself is the initial setup of a build environment. After that, any time you want the latest-and-greatest you just sync the environment to the latest changes, which takes maybe a minute, and then start the build – which can take a couple/few hours. Best done last thing before you go to bed. Cyanogenmod has good instructions here: https://wiki.cyanogenmod.org/w/Build_for_d850. When you follow the instructions to build CM-12.1 just substitute CM-13.0.

A few hints:

Your version of java

The CM13 build will fail with anything other than Oracle Java 1.7.xxx. No, it will not build with 1.8. No, it will not build with 1.6. No, it will not build with open openjdk or icedtea anything. I had to uninstall Oracle Java 1.8 to prevent its being used by default even though I had manually specified to use Oracle Java 1.7.

The vendor blobs

The instructions will say at one point to extract the vendor proprietary blobs from your current version of CM13. Huh? Not too helpful there. Use the blobs that TheMuppets has pulled together on github. https://github.com/TheMuppets/proprietary_vendor_lge/tree/cm-13.0. Additional hint: It's a whole lot easier to download the zip than it is to set up a proper git environment to retrieve them. Using git is like taking a howitzer out to hunt rabbits, in this case.

Installing

Copy the newly created .zip image from $OUT to your phone storage by your favorite means. Also copy over a version of gapps for CM13 (Android 6). First, manually wipe system. Then flash your new CM13 image and a CM13 compatible version of gapps at the same time. If you don't then the upgrade from 12.1 to 13 will fail when CM13 starts. Apparently Google Play Store is launched, and if it is not CM13 (Android 6) compatible it will repeatedly abort during the upgrade process. You can find several configurations of gapps for CM13 here: http://opengapps.org/. The "mini" for Android 6 works great for me. If you do this then it seems possible to do the upgrade "dirty"; that is, wiping cache and delvik but not apps and data. That saves a whole lot of time not having to restore that stuff. (Yeah, I can hear the incoming rounds now.)

Rebuilding after first build

I didn't use ccache at first, hoping to avoid additional complexity the first few builds. However, it works great, reducing rebuild time to just over ten minutes on subsequent builds after its first use. Use ccache.

Also, I created a super-simple shell script to initiate a rebuild. It's run from android/system/

#!/bin/bash
# sync with repository and initiate rebuild using ccache
# see https://wiki.cyanogenmod.org/w/Build_for_d850

repo sync
source build/envsetup.sh
breakfast d850
export USE_CCACHE=1
prebuilts/misc/linux-x86/ccache/ccache -M 50G
croot
brunch d850

echo "end of rebuild script"

Good luck, and let's hope a maintainer for cm13 d850 shows up soon!