So, it appears that the "feature" allowing Steam to use a library for game files different from the default... doesn't actually work. This is frustrating and annoying.
I mean, seriously, I *get* that Valve (and every game developer whose stuff they sell) doesn't actually think about anything but DirectX development. I understand the engineering and business reasons that drive those decisions, and I fully understand that they're really just re-packaging software built for DirectX under freeware compatibility layer (mostly WINE, sometimes DOSBOX, very occasionally something else). I'm even willing to pay them, so that I don't have to do the fiddling about with making the compatibility layers work, myself.
What I find offensive is that they're *so bad at it* that I end up doing the grunt-work (redirecting file locations, setting environment variables, etc), even after I've paid them to ostensibly do that stuff for me. I mean, if I wanted to hack away at WINE to get a game to run, myself, why am I paying Valve? And why on earth do they think it's acceptable to include, for example, a feature that allows me to specify alternate file locations, when that feature doesn't work? This is particularly stupid, given that the (actually quite simple) work-around has been well-documented, for two ♥♥♥♥ing years, on some dude's website. (Which is where I discovered that the answer simply is "This Steam feature plainly does not work on MacOS.")