Found an interesting little built-in After Effects preset I didn't know about while I was working on a recent project. Utility > Alpha From Max Color does pretty much what it advertises, but it's interesting and potentially useful in a few scenarios. Most likely of which is using it for creative applications rather than any keying endeavours (it's definitely not useful as a keyer for bluescreen/greenscreen footage). In particular, creating really quick alphas from rendered 3D sequences, or quick isolation/reduction for the purposes of adding an overlay for color correction.
Someone really needs to go through and do a 30-sec video for each one of the After Effects preset utilities, I'm constantly coming across ones that do exactly what I need but often not until months/years later.