Ley, you just write one message and set up a mailing list – put all the recipients in one line.
It’s not too complicated; it’s just showing respect to the participants, and it doesn’t take long.
Writing in the topic also works, but it’s still a bit worse than the first option – there’s more unnecessary information in the tournament topic. You can also create a first post on each page, like in “the best map for the third game,” but that takes up traffic and time when studying the topic...
Anyway (don’t think I’m criticizing you), I suggest thinking about this before the next offline event.

Added 7 minutes later
....and we discovered a map with Amber – I’ll say it directly: it’s TERRIBLE :D :D :D. A week (not in-game, don’t get me wrong) of cat and mouse...