You should be able to use React Portals to accomodate this use case.
To use React portals you need a DOM element you want to mount your component to. Since Express does not have one where you want it by default, we can create one like so:
WebViewer({}).then(instance => {
const ele = instance.iframeWindow.document.getElementsByClassName('HeaderToolsContainer')[0]
const div = document.createElement('div');
div.id = 'login-banner'
div.style.height = '50px'
ele.parentNode.insertBefore(div, ele.nextSibling)
})
This creates an element with the id login-banner inside the UI:
Now inside our React component, we can grab that element to mount our component: