My solution here would probably be something like setting the background of the docked "announcement" (i.e. toast) view to transparent, having a message label that's hidden/shown as needed, and set the pointer-events to none (to allow it to be clicked through if it covers some other part of the display).
I do something similar for a "quick nav" bar where I just have a few icons with no labels that quickly navigate to commonly used views. The icons then use a pointer-events specifier with !important to allow them to be clicked while allowing the rest of the docked view to be "passthrough."
To give you and update the has selector still need to be manually activate in latest release (120) and it's also working. the has selector is planned to be fully supported in release 121.