Is it possible to group an RTC Kanban plan by a custom field of type "contributor"?

Our work items have a couple of custom fields of type "contributor", where we can select users who play particular roles in the ticket.
We are experimenting with presenting our current state as a Kanban board. Currently we maintain this in poerpoint manually, and save an image an image to a dashboard every day. (Just while we sort out exactly what we want to display). I've had a go at using a Kanban view of an RTC plan, and this can do most of what we want, but I would like to group the tickets into rows by one of these customer fields of type "contributor".

In the Project Area editor in the Eclipse client, I've found Process Configuration (tab) -> Project Configuration -> Configuration Data -> Planning -> Plan View Elements -> Groupings. However, when I try to add a new grouping, I can see one of our custom fields of type "enumeration", but not any of the custom fields of type "contributor". (All out custom fields are already added to the "Plan Attributes" page of that editor, which enabled us to see their value on the plan, but not to group by it).

Is there a way to group a plan by a custom attribute of type "contributor"?