Anybody know if it is possible to call a RESTlet from a UserEvent script?

it's possible but need to send authentication headers, unlike client script restlet call where it inherits the logged in session authentication. https://blogs.oracle.com/developers/post/how-to-call-a-restlet-from-a-script-without-the-authorization-header

You can just use

N/https

and

https.requestRestlet

-> https://docs.oracle.com/en/cloud/saas/netsuite/ns-online-help/section_159139340774.html#https.requestRestlet(options) Authorization headers are not required since they are added automatically.