Postgres is learning logical replication that doesn't have these restrictions. I sure hope it can make 9.3 -- there's still a lot of ground and risk to cover -- and maybe that will deliver hope of what you seek.
In addition, the archives (base backups and archived logs) would also have to be exposed to deal with clients that may have been disconnected for a reasonable period of time, and using a direct interface to that at the storage level is not something I think is very pleasant or maintainable, because I want the freedom to change our archiving layout as we see fit to enable disaster recovery/followers as-is. I think converting those archives to streaming protocol traffic is the way to go, but this doesn't handle the issue of base-backup propagation, which is a serious blocker.
So, there's a lot of work to do all around...if anyone thinks this general kind of work is really interesting and perhaps would like to work on it on Heroku's behalf, he/she might want to consider emailing me at daniel@heroku.com.