The problem I see is, that in this JNLP file you only reference the needed files by filename. But not all files (especially the soapui snapshot file) include the version in the filename. So I assume that WebStart will not recognize an updated version of those files, right? I further assume that we will need to customize the filenames of all files in the lib subdirectory to include versions or timestamps and then modify our JNLP file accordingly.
as far as I know, webstart makes a HEAD request to the server to see if the file has been updated (in comparison to a cached size/lastModified value), so this should not be required.. (this seems to work for our existing snapshots at least).. so if you set up a local webserver with an adjusted snapshot jnlp / distribution, you should be all set..