I'm faced with an almost identical requirement. I have a working copy but I need to share my commits through a bare repository. Without the fuss of resorting to a cloud service.
I could really use a checkbox to easily create a bare remote repository when adding a new remote repository to an existing working copy or a button to push to a to be created bare remote repository stored on a local/usb/network disk.
Why do I need it? When the computer I'm working with isn't connected to the network, I create a bare repository on a usb drive and I push/pull my changes to/from that usb drive. It's a simple and easy way of moving commits from one computer to another.
Simple projects are also easily shared using a remote bare repository on a network drive.
Currently, I use msysgit shell just to run "git init --bare /remote/repository" then I add that remote repository to SourceTree and push the working copy to that remote repository.
If at anytime, I have to go find out how to do something in git command line because SourceTree doesn't support it, it is a mark against it. The simple use case that led me here was as follows. I created a repo out of a local small directory of scripts. Then, I want to push to a shared drive for others to access. This apparently can't be done (easily) from SourceTree. When I tried this, I ended up with my local appearing like the server and my shared drive as the "copy". It seems, if I created the bare repo in the shared area first, this would work out fine. I guess I will try that.