One interesting feature could be linking issues in other tracker by prepending the repository name to the notation: tortoisehg/thg/
#470. Which in this case, would become something like:
tortoisehg/thg/#470. With a dash if it was closed.
An interesting complement to this, is that, an issue created inside a user/team, would preferentially reference a repository hosted by the same user/team. thg/
#470 on the "site" team, would reference site/thg/ #470 instead of referencing the one in the previous example.