Thanks to the Witcher discord member manjack33 I was able to figure out how to do this.
First edit the geralt_player.w2ent in
characters\player_entities\geralt\ (you may need to disable all DLCs and reload the editor before it'll let you do this). After that go to appearances and add a new appearance and give it a name
Then highlight the appearances node and go to "node properties" and set "forced appearance" to the name you set earlier.
Then you should see your new appearance after saving. However you'll still see Geralt's head.
I have no idea why this is. My best guess is that you're seeing Geralt's beard option. I currently have no idea which entity this is set under. My fix for this was to go to
items\bodyparts\geralt_items\heads\ and set each head to not visible.
Hopefully someone will figure out a more elegant way to hide Geralt's default head later. But thus far this works really well. I was able to do this for example.