Yeah the moment you require different texture offsets and scalings it just turns into a mountain of work and number tweaking.
Masking without the whole blend mode shenanigans would be very cool, but I would even be fine if sprites just had a crop top/bottom/left/right property. That would entirely solve the issue here. Maybe it isn´t even that hard to implement, but that one would be up for Ashley to decide.
I´ve also checked out some video from Runeterra to see how they did it and they absolutely use two different images. It´s very quick and easy to miss. So as far as I can tell, they actually do not expand the image in the container.
EDIT: Image...