I tried exporting a clip with an alpha channel out of Motion a couple of different ways, and I couldn't get it to create a clip that was 29.97 fps.
Ultimately I want a quicktime clip in the animation codec, 1920x1080 @ 29.97 with an alpha channel.
I tried exporting through compressor settings where I forced the frame rate to 29.97 in the settings. I got a clip with the proper number of frames, but the settings showed it at 6.17 fps. Another time, it was 5.89 fps.
This caused the software I was trying to import the clip into to read the 10 second clip as a 2 second clip.
Ultimately I exported the clip out of motion in prores 4444, and passed it through After Effects to get my animation codec, but there should be an easier way.