Percussion feature request (I think)

Essentially, yes. Though there are multiple entities that need including.
Obviously this is undocumented and only for the experimentally inclined, but the userlibrary file contains (mostly empty) XML entities for most of the things Dorico defines. So, yes, you can add altered copies of the relevant xml from the app’s own definition files (though it’s probably best to give it a new name as a separate entity). Doricolib files are just another way to do that, using identical XML structure, but keeping it contained in a separate file.
I realised that the .doricolib files I’d seen had exactly the same XML structure as the userlibrary. When I was given a bit of XML for a Tenor G8 clef that transposes to put in the userlibrary, I thought I’d try making a .doricoib file for it, which worked perfectly.

I await stern words from Daniel telling us that we’re meddling with forces we can’t possibly comprehend…