Centralized credential management

In our Bamboo environment, we have over 100 actively used plans. Each of those plans checks out code from Subversion. When entering authentication credentials for the repository in the plans initial configuration stage, we use an Active Directory account that was created specifically for Bamboo (we call it a BPID, Business Process ID) instead of having the developers authenticate using their own credentials. We do this so that the repository settings do not need to be updated every time a developer leaves the company or changes his/her password. However, there are several problems with this approach. First of all, the BPID credentials are supposed to be private (not written down on a sticky note on every developers desk). Second, the BPID password is supposed to be changed every 90 days (which is nearly impossible, seeing as how it would require going into each and every one of our 100+ plans and updating the source code repository settings). I was somewhat hopeful that the new "Shared Repository" feature would solve this issue, but while it does save the authentication credentials and store them centrally, it only allows us to save one specific repository URL and does not allow for plan-specific modifications to that URL.

What I'm looking for is a way to allow all plans to specify their individual repository URLs while not having to worry about the authentication credentials. There needs to be a way to have some sort of root-level configuration in the Administration tab that allows me to say "Here's my base-level repository URL, and here are the credentials to use" and then that would let me reconfigure the individual plans to use that base-level URL and then tack on their plan-specific URL on to that one. The developer would never have to worry about the authentication piece. But then I would have the ability to keep my BPID private and I could also go to that setting in the Administration tab once every 90 days and update the password in one place and one place only.

Is this just wishful thinking, or do you think it can be done? And do you know of any workaround that I could use in the meantime? I'm using version 4.4.4.