I dislike the concept of mega-threads. It has always sounded to me like a way to drown out information instead of allow for easy access to information, and lowers the importance of the individual post in favor of whoever is more popular.

I think a better option, would be enhancing the crosspost or tagging system to be able to categorize posts by a category, and then for “megathreaded” posts you can just assign a tag. A few front ends already allow you to do usually by putting your tags in parenthesis in the title.