In a busy forum, it's difficult to keep track of all these threads for a week. If the software provided a "merge after specified time delay" feature, I'm confident that moderators would use it this way. However the current software has no such capability. I believe it's on the wish list, but I don't expect to see it added.
In a busy forum, a post will be long gone (past page 2) after a week. And if it's not, that means that lots of people have been posting in it, indicating that it is a topic with a lot of interest, and probably shouldn't be merged anyways.
In the meantime, I suggest the following work-around. The poster of any question on a mega-thread topic promises in the OP that he himself will alert the moderator in one week to merge his thread. As a moderator, I would definitely honor a request to delay merging if the poster promised to alert me a week later. That way it's easy to handle, provided that the OP keeps his promise.
Ha...most threads on common topics are started by relatively new posters. Do you really think they will know to make a "promise" along with their question?