Hi Eliot,
Confession: I'm not sure if I'm on the right track or not, but you may find this helpful.
I copied the following JARs from $BaseX to $oXygenInstallDir/lib/:
BaseX.jar
lib/basex-api-8.2.3.jar
lib/basex-api.jar
Opening a new xquery document and adding the following:
let $foo as map(*) := map {}
return $foo
does not generate any static errors.
Again, I'm not sure if that's helpful or not - it isn't apparent from the oXygen documentation how a body would add a different XQuery validator. I haven't posed the question on their forums, but I'm sure they'd have an answer.
Hope that's somewhat helpful.
Best,
Bridger