Sounds like a plan all I need is a bitcoin replace-by-fee node and the insight API I guess.
Remember, not all client allows you to sign raw tx. I suggest you to make a small tutorial(picture-only) and show them how to do it. It will be good if you give Bitcoin core and Armory compatible raw tx. Most of them don't support Armory for this type of things.
P.S. How will you relay it? I tried to push tx with almost all sites but all of them(except one - not so good) rejected double-spend raw tx.