How much of the existing PyPI ecosystem do you think could realistically work this way without package authors doing anything specifically for WASM?

In practice it is easier to support rust packages on web assembly than C or C++ packages since Rust cross compilation is in better shape. Of course packages that do IO can be a challenge but problematic IO is also more common in C libraries than Rust libraries.

Tokio doesn't work yet but epoll-based support for it is in progress.