I'm not sure if it is a Corporation-wide policy, but CBX Edmonton announced on air that one of their transmitters was going to be off the air overnight.
I heard this second-hand, so I'm not sure of the details. Most likely it was yesterday's PM Drive programming, when they were telling listeners to tune to the nested FM repeater as the AM transmitter was going to be off the air from 1-4 a.m. last night.
I'm not sure if the CBC will make such announcements where they don't have an alternative signal source. But, if you're a medium wave DX'er, it is worth keeping an ear out for, especially in the better DX winter months. Mind you, FM DX'ers last night might even have been able to capitalize on the FM repeater being off the air overnight with the conditions I was hearing this morning.