(Limited) Java extension function support in XSLT

(Limited) Java extension function support in XSLT

I've long made do with the default XML libraries provided by Java
(Xerces2-J and Xalan-J) - occasionally and directly using the latest
versions of those libraries directly when needed. It appears I'm running
up against some of the limits of these libraries - especially with Xalan-J
that is really no longer being maintained and without a release for almost
6 years...
I need to provide some custom functions to pull information from external
services when called, so they must be implemented in Java. (I.E., I can't
implement them within the XSLT itself, as either XSLT or JavaScript
functions, etc.) I've done this before using Xalan-Java Extensions.
However, providing this seems to be either allow-all or nothing:
http://www.biglist.com/lists/lists.mulberrytech.com/xsl-list/archives/200911/msg00198.html
http://marc.info/?l=xalan-j-users&m=123750821029013
https://issues.apache.org/jira/browse/XALANJ-1850
I need to be able to provide access to a Java extension - but without
allowing any arbitrary calls out to Java (think of an embedded call to
System.exit(), for example), and ideally, without the XSLT authors even
needing to know that it is a Java function (by use of
xmlns:java="http://xml.apache.org/xalan/java", for example). Ideally, I'd
also be able to keep
TransformerFactory.setFeature(XMLConstants.FEATURE_SECURE_PROCESSING,
true);.
I can do (almost) exactly what I'm looking for using
XPath.setXPathFunctionResolver, but this only works for direct XPath calls
- and I've not found a way to set a custom XPath for use in XSLT by Xalan.
(It also requires the FEATURE_SECURE_PROCESSING to not be set on the
XPathFactory, though I might be able to get away with having it set only
on the TransformerFactory - ignoring that if set on the
TransformerFactory, the TransformerFactory also automatically sets the
same flag on the XPathFactory that is uses.)
So I decided to give Saxon (Saxon-HE 9.5.1-1) a try - and am immediately
noticing 2 issues:
When just using XPath directly, XPath.setXPathFunctionResolver does not
seem to have any affect under Saxon. The set call completes without issue,
but resolveFunction on the passed-in XPathFunction is never even called
under Saxon. (It simply "just works" under Xalan.) Is there some
additional configuration necessary for Saxon - or is this maybe a
limitation of the HE version?
I've looked at
http://www.saxonica.com/documentation/#!extensibility/integratedfunctions/ext-simple-J
- which per the author, is provided for even under the HE version.
However, I can't see how to wire this into XSLT processing.