Custom Mol* extension (with inner rcsb-molstar)

[cycle20]

Hi,
I have a custom extension (with labeling and color theme).
How do I get rcsb-molstar to use this extension?

I attempted RcsbFv3DCustom constructor, but it can expect
built-in mol* extension names here, as I see:

params.structureConfig.structureViewerConfig.viewerProps.extensions:

("rcsb-assembly-symmetry" | "rcsb-validation-report" | "rscc" | "anvil-membrane-orientation" | "ma-quality-assessment" | "model-export" | "mp4-export" | "geo-export")[]

Furthermore, I guess, I should register the extension by the embedded PluginContext,
but I can't see any property to access underlying mol* viewer instance.

Best regards,
Csongor