So my super-hack works, but... it's super hacky, as expected. Also it's 53 steps long, but once you set it up once you can keep adding or removing tracks to your game freely with a minimal amount of work (import & convert new files, check a couple boxes, and regenerate a single soundbank). Though it may function at its core, it does completely replace the existing radio, but keeps the same basic functionality with a couple little quirks that some polishing can smooth out.
I'll explore the option of just replacing existing tracks that are already set up in my next adventure. It should be easier and the in-game functionality will be more familiar, but it won't be as graceful under the hood. It'd be a good one-time mod but it wouldn't be feasible for anyone planning on actively rotating songs in and out of the game.
Short version: My theory was right, this can happen, but it's a matter of getting the audio SDK assembled in a manner that the copyright lawyers will be comfortable with. I couldn't do this very easily (if at all) without Wwise.