Transport jets don't have Vne. they use Vmo/Mmo. The mo is Maximum Operating. And most people don't run their plane at barber pole.

If you're in Mach Hold it'll try to keep whatever mach number you've set. At some point coming down a cruise mach number will exceed Vmo. I don't use the speed hold functions much so I can't say for sure what would happen. I'm willing to bet most transport jets won't hold .8 at pattern atitude unless they're going straight down so I think the experiment would be short lived.