I found this[1] section in the documentation on interoperability within the JVM. It looks fairly straightforward to work with. I'm not sure about FFI more generally.
Enforcing and incentivizing collaboration also seems like a good argument for licensing open source projects with the AGPL. It prevents people who are not willing to contribute things made using your software back to open source from using it.
C can compile to WASM, but I have no clue about external tools like BLAS. I'd assume that things like Numpy are able to work without tools like BLAS, but you might also be able to compile the essential ones to WASM as well.
[1] https://doc.flix.dev/interoperability/