I think it'd be fairly easy to put together a v1 quality script hosted on an AWS instance to do that, provided the subdomains have been pre-provisioned or you're ok with them all spinning up on a single instance.
Now that Amazon S3 provides Website Endpoints and index documents [1][2], it is probably fairly trivial to set this up either using an S3 sync tool, or an S3 mount such as ExpanDrive, without resorting to CloudDrive or EC2.
You would need to set-up CNAMES for your subdomains though, and it will not work with the primary (non-www.) domain.
I would like a /sites/ folder into which I can drop webpages and have them autohosted. It has the following structure:
/Sites/
-->/root/ <-- primary page @ in DNS
-->/site1/ <-- Subdomain1
-->/site2/ <-- Subdomain2
-->/siteN/ <-- Different DNS
Putting sites in /sites/ are actually hosted on AWS so that traffic surges can be accounted for.
etc...