Making a creature lay down

You can make a creature lay down by placing the following two commands on a dialog node:

ga_play_ca_snd with the sAnim set to "laydownB"
ga_play_custom_animation with the sAnim set to "proneB"

Make them stand up by using:

ga_play_custom_animation with sAnim set to "standupB"

Note that this only work for NPCs you can select via a tag.

Thanks to John B. Gardner's Animation Viewer, where I figured out how to do this.