The only way I can see it working is if there were some sort of history being recorded, maybe like a database table which kept track of the original thread numbers. And even then, it would work only on threads merged after the feature was added.
Part of the problem is the merge itself--some staffers don't realize that threads are merged with the post order sorted by date, so it totally upsets conversational flow. The only time I tell staffers to merge threads is if 1) a current thread is merged with an inactive thread, or 2) single posts are merged into an ongoing thread (such as an R.I.P. thread) where there is no flow of conversation. A warning overlay before merging, warning of the dangers, could help. Threads are merged so seldom in any of my forums that it's easy for staff to forget the consequences.