Hi Elijah,
thanks for your response.
Indeed, things have changed lately in regards to the Hana MDC.
However, I found a solution to my issue, which is not production-grade, but allows me to carry on.
You can allow anonymous access to your XS Application using the XS Admin tool:
https://<your_MDC_name><your_user>trial.hanatrial.ondemand.com/sap/hana/xs/admin/
From XS Admin Tool, navigate to your application on the left side. In the "Security & Authentication", hit the "Edit" button. Now you can set the type of authentication you want. In my case, I deselected the "Form Based" authentication, and only left the "Basic" authentication enabled.
Basic authentication can now be done either by the client (Authorization Header) or within the proxy (AssignMessage + BasicAuth proxy).
Cheers,
Sven