" another approach is to go out of band"

the obvious (for any 'out of band') is to use the lower Japanese FM band (below our FM band down to 76MHz) - this is almost deserted of other transmissions nowadays (as any one with a scanner knows) and significantly receivers that can cover this band are already out there, at low cost. Enthusiast radios (such as Chinese radios, one example XHDATA 808 and so on) can be easily enabled to cover 76-108 MHz etc

The Japanese FM band is certainly the most viable of any out-of-band proposals