My mod Battery Pack creates a battery-powered version of any fuel-burning locomotive (burning batteries as fuel, and yielding a discharged battery as the burnt result).

It creates a battery-powered version of the dummy locomotives but doesn't create a battery-powered version of the actual locomotives; I'm going to have to look into that.

When placed, no magic happens, and you just get a funny-looking battery-powered locomotive, rather than a battery-powered single-unit train.

I wonder if it would be useful to supply a mapping of base prototype and modified prototype to make compatibility easier?

Yea I've never used any of these mods that create new copies of prototypes. So will have to look at how they work. If they all just apply a suffix to the names then I can do a core name partial match rather than explicit full match I guess.

I am trying to work through the handling of mod created variations.
What logic do you use to control what variations you create? As you seem to make a placement variation, but not a variation of the actual locomotive part of the unit. Both are uniquely named and are of prototype type locomotive.
So you make copy of "single_train_unit-double_end_loco_fluid_wagon_placement", but not "single_train_unit-double_end_fluid_loco".

on second thought I have to create the variations for the 2 Factorio Extended mods. So I will just create the missing ones for Battery Pack based off the placement version your mod makes.

It only makes variations for locomotives that can be directly placed, ie. recipe -> item -> locomotive.

Mostly because it creates two new recipes, a new item and a new locomotive.