This might be a misunderstanding.
Any property in ActivityPub can have multiple properties [except the few marked as 'functional'].
It does not need a special subtype.
If we have type ['CollectionPage', 'Collection', 'wd:Q201456'], it is a directory in a directory (?)
We have these nested structures in fedigeo anywhere, for example the UNESCO World Heritage or IATA Collection etc. We use wikidata (the data of wikipedia) so that anyone can use the label and infos for "directory" in any language …

I agree that AP/AS has its limitations and shortcomings, but —

I want this to work with Fediverse software.

If I choose a Linked-Data namespace not supported by any Fediverse software, it won't be.

...

I don't mind adding additional namespaces — and given AP/AS is missing fields for some information I want to expose, I'll likely need to.

A quick 2 second search on file system #ontologies did not directly yield a great standard candidate, but it did return some interesting results on #SemanticFileSsystems.

Like this paper and #ontology design:

https://semantic-file-system-ontology.sourceforge.net/

https://www.researchgate.net/publication/324605710_The_Design_and_Development_of_a_Semantic_File_System_Ontology

Today "ActivityStreams: Where do you want to go to today?" might be a slogan we borrowed from Microsoft.

The question is whether #ActivityStreams should be used - besides all the things it is already being used for - to also map to file systems.

The #LinkedData nature of #ActivityPub is generally shunned in favor of plain #JSON. That in itself is fine, as long as:

a) information still represents valid #JSONLD.

b) information models still follow data modeling best practices.

c) information models are designed with #interoperability in mind.

Not saying your approach is good or bad, just observing that everyone mapping and overloading their own app-specific semantics to the poor AS vocab looks to me a worst-practice. We can get away with it, as we made post-facto interop the poor man's accepted practice, lacking more rigorous extension process and guidance.

There are likely existing standardized ontologies.

Yes, I am creating a high performance read-only HTTP server for static files — that has built-in support for ActivityPub/ActivityStreams.

It also supports (the read operations of) WebDAV. So, it can be used as a shared read-only drive in operating-systems (OSes) that support WebDAV.