Official comment
Sorry, I don't know what I was thinking ... the video crossfader is assignable, it's the lighting that can't be automated. <facepalm>
The video crossfader can be used linked to the audio crossfader so the video follows the audio. This is done by having the AVSYNC button enabled in the video panel of rekordbox. The other option is you can manually map the crossfader on the mixer to the video crossfader on the software and only use the channel faders on the mixer for audio.
To assign the crossfader for video only, first unassign the crossfader channels on the mixer itself. Next, open the MIDI panel, click the VIDEO tab, then click ADD > VideoCrossFader, select that line if it's not already highlighted, then click LEARN and finally, move the crossfader on the mixer. It should pick up the value. If it warns you about already being assigned to the CROSSFADER, click the MIXER tab, select CrossFader and delete it, then go back and assign the hardware crossfader to the VideoCrossFader.