For anyone who’s following this, here’s how I’m ‘solving’ it for now. Inverted commas because this really isn’t a solution, and it’s not really a workaround either (in the sense that it won’t help if your project is all or mostly Blueprint based).
If your project is C++ and you’re trying to use SoundWaves at runtime on an arbitrary basis, ie it’s not practical or possible to preload every SoundWave via a dummy SynthComponent (Granular, custom, whatever), then you can use AudioDevice’s Precache method to get things going. Here I’ve set ‘true’ for bSynchronous (blocking Vorbis decompression), bTrackMemory and bForceFullDecompression because that suits me; YMMV. If PrecacheState == 0 (NotStarted) the audio data will be null - it’s possible that 1 (InProgress) will work, but in my use-case I can afford to block and wait for the whole thing to decompress (2, Done). Sample is a USoundWave* reference.
So, again: not really a solution to this specific framing of the problem, but maybe an option for anyone who’s desperate and C++'d up.