We could do a WASM FFI target. I have thought about it lol
My only concern is that temporal_rs packages it's own time zone data, which may make the WASM package a little heavy, so I've been inclined to leave the polyfill up to fullcalendar's implementation.
Diplomat has a wasm backend so it would even be really easy to produce a WASM ffi target with idiomatic JS and TS bindings.
Also, Diplomat supports traits and callbacks so you could actually make the timezone impl pluggable. Though we don't currently have JS support for that.
My only concern is that temporal_rs packages it's own time zone data, which may make the WASM package a little heavy, so I've been inclined to leave the polyfill up to fullcalendar's implementation.