The Blackberry APIs allow one to insert custom menu items (and corresponding event handlers) into their standard email, SMS and Address book context menus. Is there any supported way to do this with iPhone APIs?
Iphone – Does iPhone provide any API for inserting menu selections into e-mail, SMS and Addressbook context menus
No, Apple doesn't provide these kinds of hooks. However there are some tricks I've seen - one example is what Twitterific does to tweet pages that you are viewing in Safari, via Bookmarklet which uses a Custom URL to launch Twitterific.
Perhaps you can do something similar by having a text containing a custom URL. I've never tried this but it's worth experimenting with.