From this FAQ,
Flags to close a question will be dismissed as helpful as soon as another user votes to close it. However, if you don't have enough reputation to see close votes, it will appear as if nothing has happened until more users vote to close and the question is actually closed.
From Glorfindel's comment (deleted, merged into this post)
... others voting to close it with the same reason as your flag, or the question is finally closed, it will be considered as helpful
Assuming the latter is up-to-date, if I raise a flag for a reason of Blatantly Off-topic, since 3Kers do not have that option (but a custom OT reason), then is my flag still able to be dismissed as helpful because of a close vote of the same reason? (I know it'll be helpful if the Q's finally closed.)