All instruments contain errors, and the Machmeter is no different in that respect. So, yes, there is a correction between IMN and M.
The main error terms in the machmeter calibration equation (which rather defeats my efforts to reproduce as plain text in Pprune) are pitot error (due primarily to shockwave formation across the front of the pitot and high subsonic, transonic, and if you happen to fly Concorde, supersonic speeds), and static error, which tends to be proportionally greater nearer to M=1 (from either direction).
Rather surreally, the (military) course notes that I have describing this also say that North American NATO, and European NATO use different calibration equations for Machmeters.
I would have thought that if you are flying an aircraft that routinely spends time above M=0.7 either the Machmeter is sufficiently accurate that you could treat it as IMN=M, or there should be a correction chart in the operating data somewhere.
G