Not really. Mods have the ability to modify posts, what they are asking for is something like conditional mod power -to modify posts- applied to certain posts (the first post in a topic).
The main problem is not the ability to modify posts itself (because it's already created for mods and ads), but the permissions to do so.
It'll require quite a work to create a system in which the forums ask the creator of a topic which users are allowed to modify it, and then only allow those guys to do so.
Besides, it'll also make the forum more heavy to store. I believe cleod has no memory issue, but nevertheless is a problem.