Unfortunately no, that's not possible at this time. In order to map usernames (as in, /proxy/username) to the correct SHOUTcast/IceCast IP:port in realtime, an internal proxy is used to perform that lookup and then instruct nginx to do a redirect to an internal location.
While passing around a bunch of query variables would be possible, it'd require a bunch more code and effort to do it safely and wasn't deemed to be worth the effort given how few clients would use it. You're welcome to post a feature request though, in case we underestimated demand for this.