We use Bitbucket for code review where I work. So I very often go from a changeset in TortoiseHg to the changeset in Bitbucket to add a comment on it. It would be convenient if there was a link for this in TortoiseHg.
The feature is not tied to Bitbucket: just like you define URLs for arbitrary bug trackers, I'm imagining that one could define the target for changeset hashes.
I think a changeset hash in a commit message should keep linking internally in TortoiseHg. But when I am at a given changeset, the line with
Changeset: <revision number> (<hash>) <description>
could make the hash into a link to the main repository.