Hi Phil,
Unfortunately, at the moment the only "official" approach would be to build from scratch or find a suitable JavaScript crosstab library and enhance that.
Even if there was a "hack" to import JAR files I don't think this would be supported by SAP. In the case of the standard DS Crosstab, I think it is actually a special case that is coded in Java which is not really compatible with the SDK framework anyway.
Regards,
Mustafa.