Okay so you can not use Substitution tables outside of MultiScreen and MiniME function. Other Buses are not supported at this time.
However you could make a DashBoard Custom Panel in Version 8.1 that will use Visual logic to do this for you. Basically it will track the Bus and anytime that source is on the ME PGM Bus then it will change the MiniME PGM Bus to be the "alternate" source. Then when that source is removed from the PGM bus it will then set the MiniME PGM back to the Switcher PGM Bus.
It is really easy to do -- I will try to make a quick video and post it so you can create your own. Note that the DashBoard Custom Panel would have to be running on the PC/MAC or else this will not work since it is doing the tracking of this information.
#Carbonite