I wonder what it'd be like to shoot through a 600mm lens with a 2x extender...

f/20 could be a bit overkill. The sweet spot in terms of sharpness of almost any lens is usually at f/8 to f/11, and most lenses can focus the moon with those, so if the theory is right, you should be able to use much faster exposure times. I'm fairly certain that the moon does travel quite a long distance in 1/15s. However negligible it may be.

Right now I'm pre-processing my photos, so I'll be back when I've fully edited them.

Btw. It looked like the full moon was last night (maybe in the morning hours), because as I was shooting now, the right side of the moon had started to go into shadow.