From what I’ve read in the docs and this site, it seems that there’s built-in support for global maps by country name or ISO code, and for US states. But, as far as I can tell, if you want a map with custom polygons, you can’t use any of the existing choropleth support. Rather, you have to roll your own from more primitive features.
It would seem feasible to extend the current implementation to add scope=“custom”, with a reference to geoJSON data or a geopandas object and an attribute to plot. Is any such enhancement in the works?