I'm sorry to say this, but Scheme in the browser may just not be mature enough.  Javascript is surprisingly pleasant to use these days, so it might be better to use that, at least for now.

But before we make that decision, we should decide exactly what we want to do in the user interface.  For example, do we even need a web application that runs code in the browser?  Or would a standard request-response model be sufficient for what we're trying to achieve?