Uuuuuugh.

The way I have the fluid in the bottle set up, the whole fluid is always drawn. However, anything above the fill line has a 0 alpha value so it doesn't actually draw anything. Turns out I forgot that it will write to the depth buffer anyways, meaning that the invisible fluid was preventing the inside of the bottle from being drawn. This means I'm probably going to have to do some sort of fancy stuff with either the depth buffer or more render targets in order to make this work. Guh.