OK, how about this:
Whenever threads get merged (which isn't that often, right?) can you - or does Xenforo let you - somehow change the listed author of the first post to a 'shell' or dummy username with mod privileges (i.e. cannot block or be blocked), in order to avoid this problem?
For example, if I start a thread titled (+) Cavaliers in 1e and then someone else starts the same thread because they've heard about (but can't see) mine, when you merge them you'd also change the OP to show as coming from user "ENWorldGeneric" rather than "Lanefan". As nobody would be able to block the ENWorldGeneric account, everyone could then see the merged thread.