Using hgview on Debian Sid (Python 2.7.13, Python 3.5.3, Mercurial 4.0, hgview 1.9.0), I see, in a burnt orange colour, markers for branches such as default/default and mainline/default which appear to correspond to my understanding of the remote branch set up. Using hgview on Fedora Rawhide (Python 2.7.13, Python 3.6.1, Mercurial 4.2.1, hgview 1.9.0), but with the same hg-remotebranches plugin, these are not shown.
I am using a clone of this repository and in ~/.hgrc the line:
remotebranches = /home/Checkouts/Mercurial/Hg_RemoteBranches/hg_remotebranches.py
in the [extensions] section. Clearly it may be nothing to do with this extension, but given it is the same version of hgview, with what should be equivalent standard version of Python (except Python 3), raising a bug here seemed like the right first step.