Just to close out this thread (also in the thread on the users mailing list) it turns out that the JDIC JAR's manifest does not declare the JAR to be an extension library, so the application Markus is building cannot refer to it that way. I assume the same is true of the other JARs he was having trouble with.
The JavaDoc for the Manifest class has a link to the manifest file format that explains in good detail how the extension library's manifest must look and how another JAR's manifest can refer to the extension.
Thanks to Markus also for kindly opening an RFE issue
https://glassfish.dev.java.net/issues/show_bug.cgi?id=4787
for GlassFish to issue a clearer message in this situation.
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