Is it possible for heroku postgres follower database to follow non-heroku postgres master?
It's not supported in either direction, with the most pressing problem being the platform and architecture dependence of that replication method.

In the case of migrating to the service, the problem is doubly so, because slurping random, physical base backups from the internet as would be required to set up replication as it is seen today is a somewhat ticklish sounding proposition.

It sucks, and I'd like to lift that restriction some day, if that and the other problems (some not) can be solved.

Please feel free to write into postgres@heroku.com if you want to discuss your problem in more detail, although I'm most unsure if we have the technology to make this a super-easy proposition right now.  Still, the main way we can make it better is by studying it, if you'd be so kind to share.