"but I'd hate to see a lot of time spent on something of only passing interest to the questioner."

Well in a sense the original questioner isn't necessarily the only one that matters. If the replies to a thread indicate that other people are keen on seeing a song tracked down, I'd see that as good enough.

And that's another reason why it's good for people to post to threadslike that, even if they haven't extra information - to indicate interest, and in the process to keep the thread alive, and the chase active.