Where is "Back up my data" stored? Can it be edited? Anybody know how to get to it? It's not on the Google Dashboard, or linked to from the developer docs. I've even dug through the code, but it's confusing. Anybody know a way to download it, and better yet, upload a replacement?

For context: My phone sometimes starts acting weird (random hangs and/or reboots), and one theory is that some part of my backup is getting periodically corrupted, because even with a hard reset some problems persisted, but went away when I cleared "Back up my data" before doing another factory reset. Now that things have started to get weird again, I'm wondering if I could download that backup data and keep just certain parts of it.
There isn't any way to get to it. It is basically turning on and off a developer facing backup feature. Each app can write what it wants to into a certain amount of space, and Google takes care of sending it back to the app when it is reinstalled.

Even if you were to write an app for it, you'd only be able to access the storage linked to that developer key, not the other apps that use the service.

For more info on what that option does, see my answer here.