It seems that it’s possible to save remote webstart file locally. And while still using remotely distributed jar files for the webstart, modify at least few webstart parameters in locally saved jnlp -file.
However, this trick seems to work only with JDK5 version of webstart, not the one in JDK6. JDK6 seems to dutifully retrieve JNLP file from remote source, even if application is launched using locally saved JNLP -file. Thus, this trick has very limited use…