I was able to get some time in the past couple of nights to get this feature in. This feature replaces symmetry mode in the current version of the toolkit. However, I wanted something a little bit easier to work with since this is a modular system, so I came up with a new tool for it.
First, I planned out what the interface might look like in photoshop:
This is just meant to give a quick direction to start working towards. Here was my initial replication of that design in Qt:
This image doesn't show all of the iteration up to this point. The first thing I had to figure out that I didn't put any thought into during the initial brainstorming was how to list the modules. This tool is supposed to show you the modules that can mirror and let you decided how to mirror them. At first, I had a list of all of the modules, but that doesn't allow you to choose how and what to mirror. So the image above represents the next step, which was making a list of pairs: the module and the mirror module, and allowing you to select the one that the tool would use the transformations of. However, there are a few issues here. The Left to Right and Right to Left buttons are no longer needed since you specify the mirroring behavior in the list. Also, the list needs to be wider to accommodate longer names.
Here is a look at the final version(though I'm not quite sold on the color scheme yet. That might change):
Here's a quick video showing the tool in action: