This isn't specific to supporting COTS systems but we've used Agile Central very successfully for this sort of continuous flow work. We've moved all of our product development work to a scrum or scrumban style but we have lots of continuous flow (Kanban) teams happily collaborating with scrum teams.
Basically we create work items in the backlog ad-hoc, continually prioritize them and then work them through a Kanban process. For those teams we focus on metrics like cycle time, lead time and flow efficiency since they don't have a timeboxed plan to work to.
Not very specific I know, but frankly we found these methods quick and easy to adopt. I don't mind having a quick conversation but it sounds like you might be getting some attention more directly from the CA source. Feel free to reach out if you think that it will help.
-Eric