Freezing/repeating the last frame of a video clip in Apple Motion 5
Okay, figured out how to do this. Make sure you location is at the frame you want to freeze, hide all the other layers in Motion to just show the video that you want to freeze.

Then go Share menu > Save Current Frame. Then save that image somewhere.

All you have to do then is import that image back in and place it again at the end of the clip it came from. Ensure it's lower in the layer stack so that other clips appear on top of it until they disappear - or just crop it.