The idea of having a communal Xith3D webstart extension has been discussed previously. There were a few issues:
a) what if someone has custom modifications
b) what about bandwidth
c) what about the code signing certificate?
Well, the java 3d mob are using java.net for their webstart extension - https://j3d-webstart.dev.java.net/
so I guess this means that this is an acceptable practice, even if all of the end users will be downloading the extension from there (for a popular application this will be many). This solves the large problem that is b). As for a), well you'll have to keep doing what you are doing now - a custom distribution.
That leaves c). It would be very nice to see a trusted certificate used to sign the .jars, especially if one is distributing with a trusted certificate. The problem is that I don't have access to one (and most likely I would be maintaining this extension as I do the current JWS demos).
Would would you like this? If so, would you be prepared to make a small donation (e.g. $10-20) to the cause?