I also don’t have much knowledge here, but I don’t think much.
I guess the wavelength differences on HF and below would be too large for something of larger bandwidth. And anyway, the AM broadcast bands aren’t that large anyway. Plus there would still be too much RFI from foreign broadcasts.
My idea is, it could perhaps be upgraded to DRM. But this is used very rarely since it requires DRM-compatible receivers.
But personally, I don’t think this would be a better idea than AM. Having to get new receivers for over-compressed digital audio? Nah.
Second, probably not the best idea, some of those could be license-free bands. E.g. for LoRa. Imagine Meshtastic on HF.