Oddly, if I remember to change the @version on the XSLT stylesheet to 4
and the engine in the transformation scenarion to Saxon EE 13, hash()
works in the XSLT stylesheet I want it for, while in the XPath/XQuery
builder, your example does not work.

I get told there isn't a 1-argument hash function. Which is a surprise.
The example works fine in BaseX, so I'm going to presume there's no
syntax issue introduced by a helpful mail transport agent or my
ineptitude.

I haven't gone and adjusted anything in oXygen about making EE 13 the
default for XQuery, which would be my first guess as to why selecting it
as the engine for the query isn't efficacious. (oXygen has been
restarted.)

I am using the latest oXygen 28.1, so hopefully that's not it.

At some point in oXygen I used a Saxon config file to enable XQuery
version 4, though that config file option doesn't currently seem to be
enabled according to the options dialog.

So not sure whether that is different in our oXygen configurations, my
version is also 28.1.

Perhaps some other oXygen user (or developer) can chime in how to ensure
you can use XQuery 4.0 in the XPath/XQuery builder.