Are EBS snapshots not block-level atomic? In theory you should get a PITR image without stopping anything, assuming that:

1) The file system correctly orders or journals operations (I'm not familiar with XFS, but this is the case with FFS2/FreeBSD, ZFS, ext3/4 journaling, etc).

2) The database system correctly orders or journals operations, and properly fsync(s) to disk (which postgreSQL does)

Of course, there's no harm to an abundance of caution with something like this.