Now that I've posted this, the right sidebar "More Like This" includes this question at https://discussions.apple.com/message/18560201#18560201, to which Jimzgoldfinch acknowledged with his suggestion (6/4/12), "if you readd to your library files WILL BE rematched… Solution? Create a separate library that is not linked to imatch."
Any other suggestions? I'd like to keep this simple, all in one library. Evidently I want a track-specific option to "Exclude from iMatch."