Is this also called a scraper seal? I did mine by making one of those hacksaw tools described in the Moss catalog (p 81 in the TR4 catalog, probably somewhere around there in the '6 version).
I used something sticky to help hold the clip in the hook of the tool -- maybe contact cement?
Not too difficult -- certianly easier than pulling the window. Worst that can happen is you drop a clip. Easy to retrieve if you alrady have the door panel off... or you can tie a bit of thread around the clip to hang on to it if it drops, then when it is seated yank the thread out.