~/Library/Application Support/TextMate/ TextMate.app/Content/SharedSupport/ /Library/Application Support/TextMate/
For just about any OSX application it will look inside the application first, then the root Library, then the user level Library. Each one overrides the previous one. To have a custom bundle just change an item in it; they will be placed in your local Library by TextMate. It will never save anything to /Library. As for making a custom bundle that's fine... but I'm not sure of the precedence on that. Only the same names override each other, in another bundle they would both remain active and Allan will have to say which would win...