Andy,
My company is willing to take on this development as part of our project's requirements. In addition to limiting who can start a workflow, we want to prevent users from starting two workflows for the same document (Perhaps by preventing workflows from starting on checked out documents).
A quick search of JIRA didn't turn up anything, but I wanted to make sure that someone wasn't already working this. It seems like a key piece of workflow functionality, particularly if the workflow performs some actions with privileges that exceed those of the initiating user.
If I don't hear anything in a couple of days, I will create an issue and start looking at the problem.
Regards,
Aaron