For example, if a SURL is in SRM_SPACE_AVAILABLE status, a WebDAV GET on that SURL returns 503 instead of a more appropriate 409 Conflict.

On it.grid.storm.gridhttps.webapp.StormStandardFilter modify the catch clause returning an appropriate code.
Use the getExceptionReason() method to decide the right code.

The returned TSRMExceptionReason value, in fact, can be INTERNALERROR, TOOMANYRESULTS or SRMFAILURE. In our example case, adding a FILEBUSY value should permit to return an appropriate http status code.