I wonder if it is okay to summon an expert to an overlooked thread that falls within his specialty and might interest him. Also, sorry if I used the META label wrong.
A thread in this sub? Of course! That's what we're here for.
I always appreciate it, even if I can't actually answer the question. When that is the case I always try to find another flaired user that I think would have a shot at answering it and send them a PM.
Others may have different opinions, though.
I feel like a PM is a lot more tidy and in line with the subreddit's style than simply commenting "/u/AskHistoriansExpert might know more about the subject".
This is perfectly fine, though keep in mind that I believe users will only get a notification if they have reddit gold. We encourage our flaired users to use IFTTT to catch keywords for questions for just this reason.