On the workbench, when in the commit tool you select a subrepository that has local changes, you can see those changes in the diff panel. Also, at the top of the diff there is a link that says "SUBREPONAME(is a dirty sub-repository) open...".
When you click the "open..." link a new tab is open on the workbench, showing the repository history for the selected subrepository.
The problem is that the revision that is selected by default is not the "working copy" "revision". Instead tortoisehg selects the current revision.
I don't think that this is what users would normally want. It seems that the user will normally want to look at the working copy changes which make the subrepository dirty.