One thing I am thinking is, DependencyRegistrar gets called only on App Starts. That means it's called once per application pool recycle. And as far as I can see, uninstalling plugins doesn't restart apps, which means it won't work like how you want it to even if you write it inside DependencyRegistrar. Unless I am missing anything, I think writing inside the method is needed.
So your method will check if plugin is installed: if yes, run code; if not, call the original nopCommerce interface / method.
:P