If you do not use exorbitant amounts of propane to accomplish what a small amount will do, which is simply help burn the diesel that is present more completely...., then yes you can actually see lower EGT's. .

Now mind you, this is not a tremendous drop by any means, usually only about 50-100 degrees but it should not cause them to increase.

And yes, you are correct, propane is not "technically" or by definition a "catalyst" in the true sense of the word, but for lack of a better way to describe it's effect on the diesel fuel, it is used to explain what happens.
So while you are adding a fuel, the key is to not add a lot, (i.e. control the amount) so that you burn it (the propane) completely and more of the diesel along with it, such as with a system that is variable and controls and meters the flow of propane by a constant perimeter such as ...........boost pressure

Gauges are a good idea, regardless.