Meta-tag to dereference URIs

This is an open question about ShEx extensibility via "metatags" (i.e
customs semantic actions, a la GenX).

I wish I could write a metatag to tests that my object rdf:Resources are
dereferencable.
What could be the process to write such a metatag?
May be, I could import an external library in my webpage and get the
benefit of my metatag immediately?
Or shall I hack the existing code of ShEx/Js?

Other interrogation:
My requirement will probably be implemented with (asynchronous)
XmlHttpRequest.
How could that fit with ShEx semantic actions, that are probably purely
synchronous.
Could promises/futures help, in that case?

I would appreciate all your comments, and help is very welcome.