May others find it useful:

I created a small dummy java class that implements public static String evaluate() method (with 1 to 5 arguments for my needs), compiled it and made a jar file.

replaced original references to java:com.intersys.xsltgateway.XSLTGateway by my faked ones (Atelier refactor, easy...) in all used XSL stylesheets.

Then I supplied this dummy jar file into a list of external JAR files to the OrangeVolt XSL run configuration. It all worked fine and I eventually received transformed document (with dummy data in places of original isc:evaluate calls).

Just need to remember to put back pointer to original callback handler class before deploying transformation to Cache server.

sure, supposed you have a class opened with the rule definition (the XML data you showed earlier) you just click a tiny icon with a little search glass in it. Or you can press Ctrl+Shift+V at the same time. This would bring the ObjectScript layer of the rule.

But my feeling is that condition is simply never called due to the constraint in the rule 1. the constraint is evaluated first before it let's you go and evaluate condition within rule.

You can also change level of tracing information produced by the rule, just makes sure that you production has "Log General Trace Events" turned on, then the Business process that uses the rule has "Log Trace Events: on and Rule logging set to "a" for collecting ALL details. This could also help in determining what is rule evaluating.