The https://github.com/Esri/ago-tools toolset has some interesting tools for accesing AGOL items, including updating service URLs in web maps. My guess is if the URLs can be updated, that an inventory might be able to be made also. I have just started looking at this toolset myself so I can't add too much more about it. But it looks to have a lot of potential, and since it is on github, hopefully more features will be added to it.
There is a download a zip on the right side if you just want to take a look at it (python)....if you are like me and not confident enough to jump into the cloning and forking yet.
The author says he is open to suggestions for new features. This is not an immediate solution to your issue, but is worth checking out anyway.
