Hello Gath,
This issue is most likely resulting from TopLink's use of a temporary class loader to allow for dynamic weaving. I am not overly familliar with Webstart but assume that when TopLink creates the classloader copy the jar files are downloaded from the remote system again. The solution may be specific Webstart classloader support. I would recommend looking at JavaSECMPInitializer.createTempLoader.
If you have not already done so please file a bug for this issue as well.
--Gordon
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