It's hard coded, so you cannot affect that at all. 7.3 does improve on things in this area quite a lot.
Does this mean that on my Problem Assignments tab I cannot display who the problem is assigned to? That can't be right.
Now how do I break this to you gently Oliver.....? If you upgrade to 7.3, you will find this little inconsistency has been adressed.
In 7.2, there is a way. You write a query against the assignment object and that way you can list the current group, user and role an incident is assigned to. You add this to the window and then modify the query so that it has a criteria which prompts against the incident ref. A bit of magic happens when you do that and the query has that prompt filled in at runtime with the reference number of the incident. The result is a tab which just shows the current group, user and role assignment.
But personally I'd upgrade to 7.3 if you can as it has lots of new features which are very useful.