If a built-in solution exists, then I am unaware of it.
I agree that a system variable should exist (XrefReloadAuto?), which hooks the existing FileSystemWatcher that is instantiated for the parent directory of each external reference that exists within the Document Collection.
While much is unknown (or at least undocumented) about the code-behind that supports the external reference mechanism(s), as I understand it, they (Autodesk) need only hook the DocumentaBecameCurrent event in order to evaluate the value of the pseudo XrefReloadAuto system variable (thus effectively reloading external references automagically).
In order to do this via LISP, you'd need to implement your own DocManager reactor, in order to monitor the same DocumentBecameCurrent event, but rather than employ a FileSystemWatcher at drawing open you would instead possibly store each external reference's vl-File-Systime in a grouped pair (with the reference).
When the DocumentBecameCurrent event fires, test for updates to the stored list of external references and associated systimes. Also be sure to implement the necessary Command, or AcDb reactors in order to update your list of grouped pairs as needed.
My $0.02