Editing the values in the box model is already possible. You can simply click them to change them. In comparison to the Chrome DevTools they even have a hover effect indicating that you can change them and it only requires a single-click while you need to double-click the values in Chrome.
So, what exactly is missing in your eyes? Or doesnโt it work for you?
For the search button in the Inspector Iโve created bug 1258963.
Sebastian