It’s not exactly the same, but this plugin does this already:
https://github.com/nunopress/grav-plugin-metadata_extended
It looks to targets MetaData but it might work with other headers, or with a very slight modification it might 
It’s not exactly the same, but this plugin does this already:
https://github.com/nunopress/grav-plugin-metadata_extended
It looks to targets MetaData but it might work with other headers, or with a very slight modification it might