How about using the new AWS Step Functions instead? This would allow a full push model while still keeping a nice and tight workflow:

1. fire off AWS Step function on new post to S3 bucket

2. read delay / schedule time in metadata from (assumable) markdown post

3. trigger AWS Step Function using the Wait State defined based on read value from 2.

4. As part of Step Function: After Wait State -> Execute Lambda to publish post.

Note: 3) requires setting variable time in Wait State based on environment. Haven't read up so don't know if that's possible, but guess it should be.