I'd also like to take advantage of the tournament's Custom JSON to store the file ID of a JPG image to be displayed in the game UI to promote the tournament (example: in December we'd ask users to play a Christmas-themed tournament and the image would be snowy) but I don't see how to edit the Custom JSON through the portal - is that only possible through with Cloud Code?

A little more progress - although passing a version ID of -1 to GetTournamentStatus() produces an error if a no tournament is running, I can instead call GetGlobalLeaderboardVersions() and then call GetTournamentStatus() individually for each returned version.

If runtime discovery is difficult, then I could use a fixed leaderboardId such as "monthly_event"

I experimented with that, and when a tournament is over I can call:
TournamentService.ViewReward("monthly_event", -1) and in the response see that tClaimedAt is 0

but to actually claim the reward with ClaimTournamentReward I have to pass a specific versionId - how can I figure out which versions of the leaderboard the user has unclaimed rewards for? That's particularly difficult if a new iteration of the monthly event has already started.

More generally, we're evaluating switching from Firebase for a music game with thousands of songs. In the Firestore database it's easy to have leaderboards with secondary indexes for player country/region. I don't think Braincloud supports that so we're going to need tens of thousands of leaderboards with IDs such as "song_456_US" and "song_1234_CA". For that reason, I need to avoid any situation where I have to list and filter all leaderboards because it will be (I expect) too expensive ($) and/or too slow.

I should add, this is part of our evaluation of Braincloud as a potential BaaS for our game, so I'm trying to map our requirements onto Braincloud features.

In the context of https://help.getbraincloud.com/en/articles/3248895-introduction-to-braincloud-global-tournaments and especially in the situation "If you implement concurrent tournaments, offer a means to cycle between them", how can I programatically query the current active tournaments?