Cannonball:
If you want a lot of examples in railroading, just ask if "something" never happened.
For a while Model Railroader ran a column called "There's a prototype for everything" which was pictures of all the things that modellers are told never to do.
If you go back to the transition era, there was a lot of steam-diesel helpers and diesel -steam. but technically they wern't MU'ed since there was a crew in each type of loco with independent control.
Cannonball: Diesel helpers on steam powered trains.
Steam helpers on diesel powered trains.
844, left, and 4449, right, both have MU control boxes, next to the speed recorders, for useing diesels as helpers as trailing units only.
The box above the MU unit is the train radio on 4449 and the "FRED" unit on top of that on 4449 while 844 has its "FRED" on top of the speed recorder and its train radio is over the engineers head