Phabricator also supports hosting mercurial but there are some rough edges. Its need to do some basic monitoring of the wire protocol (primarily to determine whether actions are read/write for ACL and cluster resolving) have uncovered that interpreting the underlying wire protocol is difficult and challenging [0]. I wonder if Kiln or others have similar issues? I would guess that this sort of problem is one of the reasons hosted tooling around hg falls behind git even though touting a great extension API (aside from the large gap in user base).
[0] https://secure.phabricator.com/T9548