As I already wrote, a right-click on a drive letter in MC drive bar will show the disconnect command for this single drive. So it's already possible to disconnect from MC.
Anyway, other Commander use F12 to launch the "Disconnect Network Drive Wizard" (e.g. Altap Salamander, DoubleCmd). So I'm familiar with this key. And you can disconnect several drives at once from the wizard.
As far as I found hints, usually the WNetDisconnectDialog function (
https://msdn.microsoft.com/en-us/library/windows/desktop/aa385440(v=vs.85).aspx) is used to launch the disconnect wizard but since Windows 2003 exists no more connection to the shell32.dll, so it cannot be launched from Rundll32. Instead it's related to mpr.dll.