If someone asks a (factual type) question in a comment to someone else's lj post, and you know the answer, would you post it in a reply?
Yes, it wouldn't occur to me not to.
It depends how well I know the original poster
It depends how well I know the questioner
I'd wait a bit for the original poster to respond, and reply a bit later if they hadn't
No - it feels too much like butting in to someone else's conversation