General protocol / best practices for passing rich display for kernels

[joequant]

The specific issue is that I'm working on the metakernel based C++ kernel JupyROOT, but I would like to have the C++ rich display system in xeus work with metakernel.

I can override the C++ code to generate data for rich display, but this needs to be passed through the metakernel layer. This brings up the question of what are the standards and best practices for a metakernel based kernel that wants to display rich text.