I'm sure a number of people have desired this at one point or another. If not, I fail.
Moving on. A number of people tend to work on group projects. An example I will use is two people creating a map.

Mapper #1 creates topic "My Map" and fills the topic with his ideas, plans, and basically everything required for a topic to exist.
Mapper #2 who is also working on the exact same map has updated a few things. He requests that Mapper #1 to update the Original Post.
It just so happens that Mapper #1 no longer has internet access. The thread cannot be updated.
It just so happens that Mapper #1 no longer wishes to work on the map, but Mapper #2 still has the ambition. Mapper #2 cannot update the Original Post.

Even for something like the SEN Observer. Having only one person being able to update the original post may be tedious. I don't think everyone has the time to update the Original Post with info repeatedly. People reading a topic usually look at the Original Post to find a summary. No one really wants to read through an entire thread just to find out updates from a week's past.

Basically, request is:
Allow users to designate who else may modify their specific post.

Yes/No? Or maybe I'm delusional again.

Has this situation actually happened to enough to warrant coding around a bunch of existing things?

Is it that impractical to open a new thread and ask a moderator to lock the old one?

Waste of time. If you have a partner then just have them post in the same thread. Who cares if they aren't able to modify the original post.

The partner could also copy/paste the OP into a new thread and let the old one fade away.

Would be a little nice I guess but def not a high priority especially since i've never actually seen it happen..

I just got my new thread deleted because my inactive for over a month old thread apparently wasn't locked like I thought it was....who knew we had to go back a few pages and check for that first. I guess we will just have to revive dead threads indefinitely .

I originally locked the new one because it was a wall of updated info and a link to another forum dedicated to the map. Upon realizing it was for a map that already had a topic, I deleted it. Tons of topics that are lost for months are revived due to some new interest in production, and this didn't seem any different.

Either way, this thread seems to be about group projects, so this particular case seems irrelevant.

Sounds like what you'd use a permission-based wiki for. Threads are crap for serious projects, IMO.