This is available for operation behaviors like the SCM deliver operation as explained in the second post of https://jazz.net/forums/viewtopic.php?p=47650#47650. There you choose approvers based on roles.

I think this is a blind spot still, In SCM deliver, you can choose how many approvers with a certain role are needed, but I am not sure it provides with a comprehensive list of approvers with a certain role.

This is by the way also not trivial to solve, since you would be able to come up with many different scenarios how and where to look up the approvers. One possible solution would be to add a work item attribute of type contributor, backed with a role based value set, where a user can be selected. A participant could create the approval as described here http://rsjazz.wordpress.com/2012/11/30/a-create-approval-work-item-save-participant/ and add the approver mentioned in the attribute.

If you have these kinds of scenarios and needs, I would suggest to create enhancement requests and communicate them here to others with similar needs, so that development can find working pattern.