Custom JS script on Wopiframe2.aspx

Question

In order to customize system notifications, I'm using JavaScript loaded through an AdditionalPageHead for replacing the HTML in the maintenance message displayed on top of every .aspx page in the farm. This works as expected.

But when viewing a document by using Office Web Apps, i.e. {site_url}/_layouts/15/WopiFrame.aspx?sourcedoc={document_path}&action=default, the AdditionalPageHead can't be added - as the master page is missing.

How is it possible to inject custom JS into the Wopiframe2.aspx page?