This relates to an interpretation of QM that I have not personally found much reading material on, the idea that wavefunction collapse is actually just entanglement; that is to say that as soon as a system in a superposition interacts with the outside universe, the outside universe becomes entangled with the system, thus putting the entire universe into a superposition where both outcomes are observed.

That's precisely Many Worlds! The process you describe is a pretty close match for what's called decoherence. Perhaps you'll have more luck with those search terms, there are quite a few really good resources out there. Just be careful to steer clear of the partisans. However much I disagree with them, the shut-up-and-calculate people give the best overall picture, even when describing a particular interpretation.

Is it? For one reason or another I thought there was a distinction, although reading back what I wrote a second time I can't figure out what it would be.

It's interesting to consider the implications of this line of thinking though; it seems like it would imply that causally-disconnected regions of spacetime (i.e. things outside of our light cone) are forever in a superposition of all possible quantum states, from our perspective, among other things.