Is there information anywhere about how to construct a memo function? I
see that support for it it is built into Saxon PE and EE, but does
anyone know of an example or descriptionB of how to construct a memo
function in HE, or a different XSLT engine that does already incorporate
hooks for that functionality?

To use that feature, the processor has to support it as far as I
understand, the test suite has a test case
https://github.com/w3c/xslt30-test/blob/master/tests/decl/function/function-1
031.xsl.