So on further investigation it turns out that the script API is working as expected, and the problem lies somewhat closer to home - namely in my (lack of) understanding of how this particular aspect of javascript works :oops:
1.36.257 therefore now copes with being loaded as part of a WME session started from a URL containing references to URs, MPs etc…
https://greasyfork.org/en/scripts/435828-wme-fix-ui-memorial-edition