... the people who built this engine say they are brass inserts... or at least, it says so on their website...

Next time I see the man, I'll ask him...

I wasn't reacting on what you said, I know a single cilinder engine can't be balanced completely without auxiliary balancing shafts.
I was just pointing out that positioning your (out of balance) prop in a certain way can have a good effect in some very rare case, but that it is very unlikely to get it right by just putting an unbalanced prop on the engine.