Synchronous buck converter in parallel with linear regulator
It would appear that the answer to "how to make a buck converter not suck up tons of power when subjected to no-load condition" has already been answered by manufacturers who have greatly improved buck converter no load current consumption over the years.

If you dig for long enough, you will eventually come across buck converter data sheets that specify the no-load current consumption of the converter.  Some converters have pretty darn low current consumption at no load, such as the TPS562200 which consumes ~22 mA at no load