I believe that the "tabs" permission does allow you to read the current URL without granting the ability to modify the page. It's also possible that some of the functionality could be built with the chrome.declarativeContent API or other alternatives. I'm sure there are features that would be impacted, and I doubt that most users would understand or care about the reduced permissions. But I would.
The main thing is that the extension would still have to be able to run its JS on any page to be able to show the interactive sidebar. Not sure there is a way around that, unless chrome decides to implement the sidebar api that firefox has (https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/Web...), then the sidebar code doesn't have to run in the same context as the page.
