Assuming the custom container for galleries doesn't take off, my backup plan is to use my original idea of detecting multiple images in the post bodies and populating a gallery. To make that behavior explicit, I would probably only trigger that behavior if the post title has a gallery tag/flair (eg. Check out my vaction photos [Gallery]
).
Either way, I'd like to get feedback from other app developers to see if they'd be interested in supporting similar behavior and standardizing it.