You can implement custom Tweeting interfaces either by authorizing with the REST API to submit Tweets directly for the user via your own interface, or you can use Web Intents to invoke the Tweet pop-up with a prefilled text using the text parameter, e.g.
http://twitter.com/intent/tweet?text=Foo
You could dynamically set foo from a text area when your user clicks a button. (The user will still need to click “Tweet” in the pop-up to confirm.)
