I think I'm starting to understand the madness that is modding Factorio. With how it is set up, you just can't half-ass your kludge solutions - you need to go full-ass in order to get what you're after.
For instance. I explained how the burner-generator is actually a boiler and a steamengine mashed into one place. Now, the boiler has a 'fire' graphic that shows that it's burning fuel. Since the boiler has different orientations it can be in, it has several variants of that graphic, which is nice to have. First, obvious idea - when creating the boiler through code (this works because I flipped the original code around), simply set the boiler's direction to be the same as the steamengine's direction. Unexpected error - the game does not allow setting the boiler's direction in code because it's also a pipe and its directions are determined automatically. Uh. Okay. After experimenting, I ended up having to add an entire new boiler entity, which is completely identical to the first one except that it uses a different default fire graphic, and the boiler-creation code chooses which boiler to create based on the orientation of the steam engine. Complicated enough? Nope, turns out I also need to add an item that matches the new boiler, because the game apparently wants it for some reason.
I also can't just add a burner-powered lamp (aka sconce, or brazier), because a lamp has no UI for letting you give it fuel, though it will flash the "out of fuel" icon. Wonderful.