I'm on Windows 8. I hate the use the keyboard and I just discovered scrivener and I'm loving it but I miss this option. I usually achieve that on File Explorer by Ctrl+L then Tab to the sidebar. I'm on Win 8 Pro x64.
It's not possible. I have searched a lot for this feature as well, and although I have no official answer, I have seen all of the shortcuts and this one is not there.
The best thing you can do is to navigate between documents with ALT + SHIFT + UP/DOWN. You can toggle the binder using (CTRL + SHIFT + B) if it was hidden.