I think we should explore how we support compound-CRSs, which is done by proj/gdalwarp by using 2 EPSG: EPSG:28992+5709 (this is equivalent to using EPSG:7415 but there are cases, like copdem where EPSG:4326+3855 is used and there is no one EPSG for this).

The OGC Name Specs now accepts CURIEs, and it is explained there how we could support this:

• Support for CURIEs (compact URIs) opengeospatial/ogcapi-features#632
• Question about coordRefSys and compound-CRS opengeospatial/ogc-feat-geo-json#102

this would mean modifying the "metadata"/"referenceSystem" to accept an array of strings, I guess.