A thread could work if a certain user takes it on themselves to take all the sidebets from the different threads and edit it into a new thread.
- Jay -
It doesn't need to be heavily moderated to work, it's just about the OP stating the rules of engagement in the thread in the first post, and local rule of no off topic posts that doesn't include the finalization of a sidebet by the two users. If that's been done any other post would only need to be reported and most likely will get deleted by the admins based on the local rules, it might take while to make the thread a perfect one, but I think it would work out well, just like the collectibles section.
And about the thread getting lost in middle of all threads overtime, it's easier to bookmark or enable following for just one thread. You'll also need to bookmark the spreadsheet or look for the post containing the link as well as time goes on.