Can ME with centreline thrust propellers be considered to behave like a single engine in the event of one engine failure and Vmca not being applicable to them?
Kind of. FAA has a centerline thrust rating only, should you want to do that. There used to be a few Cessna Skymaster pilots with that. The problem is finding an instructor and examiner that is current on that rating - it's easier normally to just do the regular ME rating for that reason.