Pivotal solved that by requiring everyone to work the same exact hours. It is a bit military in that regard, but it works for them. You don't get a job there unless you buy into it. Their reasoning is more around their culture of pair programming.
It sounds like you have found a solution that works for your company and that is great! The way I look at it is that as long as we are shipping software around the time when we say we are going to and the quality is high and people aren't killing themselves (weekends) to make it all happen, all good. =)
What we have been trialing for awhile is a simple Slack checkin. Everyone reads them when their day starts, and if there are any questions or comments a Slack thread is started from the checkin message. It's worked quite well.