Hi AdrianSutton.
It sounds as though you would have a mixture of new players and those that are already familiar with Carcassonne. Is that correct? If so, any kind of tournament might be a struggle as newer players won't enjoy competing against more experienced players and vice versa.
You could try doing meeple stacking, or meeple art (people make pictures out of meeples, although you'd need hundreds of them) as something to draw people in, and have a leaderboard for the meeple stacking and photo gallery of meeple arts produced.
Something else that is sometimes popular in Germany is "simultaneous Carcassonne" where a single player plays a series of head-to-head games against multiple opponents and just works his way down the line of tables, playing his move before going back to the start and working his way down again. You could offer a small prize to anyone who is able to win their game, and this might still be fun for those with less experience too.
If you contacted HiG and told them about your event, they may even send you something that you can give away as prizes. It might not be much, but it could still draw people in. Or if not, I'm sure I could get hold of a bunch of signed tiles or something if you're struggling.
If you charge a nominal amount to take part in any/ all of the events above and there is sufficient interest then it may add up to a worthwhile contribution to the overall total.
Good luck! And please let me know if I can help out with anything...