Agreed, Ctrl-G is a godsend. I don't have multiple displays set up right now, so I can't test an actual solution. You may be able to move the in-game window (only in-game) if you turn borderless off and specify an in-game resolution smaller than your desktop resolution. 

Alternative solution: You can move WA around between screens and stuff if you install it in a virtual machine. I used Oracle VM VirtualBox. You need a good computer though or it will stutter. Probably too much work unless you have a VM set up already.  You will not be able to move the frontend (menus) around or to your secondary monitor unless you use a VM.

I heard a rumour that they made some improvements to the frontend in WA 3.8. I don't know what those improvements consist of, but hopefully some kind of window support (or support for 3rd party windowed support).  We should know soon - the update is finished and is just pending Team17's rubber stamp approval process, and then it'll be released, hopefully.