Can't create a WASM library with Qt ?

ATM I can build full Qt/QML apps with Qt for WebAssembly and run them fine in a browser with qtloader.js and WebGL support, but my need is much simpler, still cannot find any information on how to do it, or even possible :

I need to create a simple WASM module that exposes a function with signature (QString) -> QString, using only Core and QML modules (no GUI, Quick, etc) and I first thought I would just need to set TEMPLATE = library in QMake project then somehow expose my function, but no clue.