You might want to look into the
Twitter API which I believe allows you to search for keywords/callbacks in their database. Meaning, a user can tweet via any means outside your site, then when they visit your site and login and provide the necessary twitter account info (say, in their profile) you can run a server side API search and see if they included one of the band's related callbacks/keywords in their tweets. If the query is true then present the download link, etc.
Am I making sense?
A quick glance at the API
documentation shows there is a query method with callback and show_user parameters so you should be able to see if users on your site posted a tweet with one of your callbacks or keywords. There are also links in the API home page for discussion groups where you could ask about this. All I can think of so far.
-jim
