Sorry for the delay in replying Nick, was on a long Christmas vacation.
Check out Multithreading 15/N: Synchronous stdio by juj · Pull Request #6082 · emscripten-core/emscripten · GitHub with the ‘paste’ handler, that implements a copy-paste handler to a text area. I can’t immediately recall off the top of my head how SDL will behave here, but a similar approach could be used, except that Module.writeStdin() does not (yet at least) exist for SDL, since that PR is only for multithreaded ASMFS.