Not the same, but definitely a related issue: Determining who has linked to one of your nodes?

"posts by a given monk that are replies to one of my posts" is easy w/Super Search, assuming that the replies have the same title: search for posts by that monk and w/the title constraint.

"posts that are replies to a given monk?" is harder (assuming you don't have a specific thread in mind to title search) .. would need to cross-reference that monk's nodes with your looking for the same thread (there could be some false positives in there, too).