Maybe I'm missing something but I cannot find the XQuery way to get access to the logged in user from a web application using RESTXQ.
The servlet request object has getRemoteUser and Exist, for example, has xmldb:get-current-user and Marklogic has xdmp:get-current-user.
We could add a function user:current() to the new User Module [1]. More suggestions on other, possibly missing, functions are welcome.
Best, Christian