off airway MEA's

I'm wondering if there is a way to determine the minimum altitude
required for uninterrupted communication with ATC off airways? Do you
look at the closest airway and sort of interpolate the MEA? Do the
center and approach controllers have maps with these altitudes? I'm
assuming an MVA doesn't carry a communication guarantee.

I'm planning a flight over Canada that leaves the Detroit area, up the
Bruce Peninsula and back into the US in northern Michigan. I ended up
calling Toronto center on the phone, but it would be interesting to be
able to figure this out on my own. ... akiley