Modify "open with" list for file type (not for duplicate removal)
I will suggest an alternative way than modifying the system's "open with" list.

I use LaunchBar as my main tool to interact with apps, files and more. I have configured LaunchBar so that when I select a document (or some text) and double tap the command key, LauchBar will allow me to specify the Application to open the file.

I believe that Alfred and Quicksilver have similar abilities.