Ironic how I originally asked the question…
Say you need a 10 second loop, right.
Start with 6 seconds of footage that you want to loop.
Duplicate it, move the timeline to the middle of the footage.
Cut the top layer Alt + ]
Zoom into the middle and check that the timeline matches and cut the bottom layer Alt + [. *After FX doesn’t seem to do this seemlessly if you’re soomed out.
Move the top to the end and the bottom layer to the beginning.
Go to your comp settings and change the duration to 10:01
Then drag the same original footage into the comp and key frame the opacity to fade in and out. Crossfading 🙂
I also apply and easy ease to my key frames so it isn’t as jerky on transition.
Does that help