cdavbar....
"Cross-shard" events do not require cross-sharding characters.
The judges, after all, don't HAVE to be fishers.
You can have multi-shard events happening simultaneously, with judges present on each forums (someone's gift swappers, for example), and make arrangements through one of the multi-shard guilds (say, someone with Whispering Rose Radio or The Syndicate, as the two most obvious choices) to have the cash prizes paid in on one shard, and paid out to the winner on their home shard.