Gateway Memory Tag, bound onto a custom property on an always present object in the sessions (docked view) that opens a popup using a value changed event on the custom property. Button sets value to true, have something automatically toggle it back off after a set time period.
or
Combination of system.util.sendMessage with handlers on projects to re-broadcast at the session level with system.perspective.sendMessage, with a handler on an always present object (docked view) that opens the popup using system.perspective.openPopup