Yeah, what @nojaf suggests above would be a great way to test this put. Doing some minor research, it seems that .svelte files are only ever imported into other .svelte files for using as components, so while that part still needs working out (%raw should be fine for now), the approach outlined above should be very much doable. As in - in a Svelte preprocessor, take the JS content, write it to a temporary file, and then compile it using the compiler command Rewatch can give you for your current project.
There are a ton of rough edges here that we’d be happy to smoothen out if anyone is interested in using Svelte with ReScript, and gets going trying these things out. This includes editor tooling as well (which we also can figure out how to support if there’s proper interest).