@dalias Ah!
Presumably nobody mentioned in the original toot should be on the outbox for the boosted toot, either. Like if you're discussing something with a third party, and page me in with a mention, and then that toot gets quote boosted, I do not want replies to the subsequent discussion, either.
Really, it sounds like a boost quote should have no live mentions in the quoted part. The toot author should be notified, of the quote boost, but only the added commentary is live on it.