This is why I am asking. If it is just you want to convert XXX.tmLanguage.JSON to XXX.tmLanguage; I don't think a build system is really needed, I can easily add it the plugin, and you can bind it to whatever key combo you want or just access it from the command palette.
Either a command or build system can be implemented though. I don't mind adding more in depth features, or even build systems. I currently don't plan on using AAAPackageDev, so having some standalone plugins to do commonly desired things is what I am aiming for; that way you can pick and choose what you want, and each one can be even more specialized in what they do, because that is all they will do. This plugin is a bit more flexible because it can do any plist you like: tmTheme, tmLanguages, tmPreference, etc.
I personally like the toggle, because I can just toggle it to json, make changes, and toggle back and save. But if you want to keep the json like a source code file, and the tmLanguage like the compiled code, that can be done by a simple command (no build system required). If you want to search an entire project, then maybe a build system would be helpful, but I still don't think it would be necessary. Build systems are better for feeding an external executable that does some compile stuff with project variables.
I will play around with some ideas, and see what I can cook up, maybe even a build system