Oops -- figured out what was going on, and it was my fault.
I had excluded *.wav files on my exports because I've been toying around with mono/trimmed .smp files. I forgot to update one of my SampleLibraries to use the .smps, so when I ran things from the editor they worked fine because the .wavs still existed, but when I exported it those files did not exist so that scene failed to load which cascaded into just about everything else failing to load, since I'm preloading everything in a global singleton.
So preloading resources as you would expect does indeed work in both the editor and exported releases: preload("res://Path/File.ext")