Score another point for memfs, the in-memory node.js fe impl (which if Im reading this right underpins your wasi fs impl). https://github.com/streamich/memfs
Edit: apologies... your reply says you have you own impl! Damn, awesome! Apologies, I'd seen memfs in a dependencies here: https://github.com/taybenlor/runno/blob/main/packages/runtim...
WHATWG recently took up File System Access spec as their FS spec. It both looks semi promising, but they seem to only care about & are only building specs for specifically emscripten wasm users. Only sync apis, only usable from dedicated workers... there's some hopes for more latter but feels super weird to see the web finally get fs access & have it be fast... but for it to be extremely odd shaped hand tailored to a very narrow class of use.