I feel it should probably be ported to this thread.
I'll quote it here:
The terms of creating Flags is quite clear in that the thread cannot be a self moderated one (this thread is self moderated) and the thread cannot be locked. You did not create the thread, therefore, you cannot guarantee that the thread won't ever be locked.
I'm pretty sure the requirements apply for creating the Flag. There's no way as a user to guarantee a Mod won't lock the thread later. I would Oppose Flags if
the Reference topic doesn't exist, not when it's created by someone else.
If theymos intended the Flag system to have a consequence when the topic gets locked, he would have automated it.