I'm not sure if you object to the aesthetics of using the wrong shifter, or you don't understand how it would work. I do admit that upside down doesn't quite describe how you would install the shifter, but I don't have a great way to clearly describe the translations and rotations needed to orient the shifter because I am bad at geometry.
If you rotate a rear shifter around so that the grip part is facing left and mount it on the left side of the handlebar, you can use it to shift the front derailleur. It will rotate it in the opposite direction of a standard shifter, but it will work fine. The only real difference between most "front" and "rear" friction shifters is the handedness. Unless you have a newer rear derailleur that requires extra cable pull, you can usually swap "front" and "rear" friction shifters. The only other non-interchangeable friction shifters I have run into have been shifters that also had indexed modes, and shorter cable pull for the front shifter. But that doesn't come up if you are using a "rear" shifter for a front derailleur.
Old Suntour barcons didn't have different front and rear shifters, and some early mountain bikes used two "rear" shifters since left-handed thumb shifters were not readily available. Once you move away from indexing, you can configure your shifters a lot of different ways.