Saxon

The XPathCompiler sets up a static context for XPath expressions which by default includes the standard functions in the XPath 3.1 function library.

Some of the XSLT-specific functions such as key() don't make sense outside a stylesheet. That isn't the case for system-property(), except perhaps for properties like xsl:supports-streaming.

You should be able to add the standard XSLT library to the static context using something like

FunctionLibraryList libs = (FunctionLibraryList)XPathCompiler.getUnderlyingStaticContext().getFunctionLibrary();
libs.addFunctionLibrary(XSLT30FunctionSet.getInstance());

but some of the functions may fail unpredictably due to not having parts of the XSLT static context available.

Not tested, and use at your own risk.

What is supported is using fn:transform() from XPath to execute an XSLT stylesheet, and of course the XSLT stylesheet can be a simple call on system-property().