Simple idea: rather than asking people to tweet the dedication, give them a list of the different licenses available, then they can choose one, it takes them to Twitter's site with the status field pre-filled, so they can then just hit submit and it'd be done.
To see what I mean, go to http://twitter.com/home?status=Foo - the status field is pre-filled, so you could just redirect people to http://twitter.com/home?status=@tweet...
The alternative would be to do it with the API, but that would mean implementing either the password anti-pattern or OAuth (etc.). A link is enough.
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