> I propose the following  [brief/ main steps of a] concrete solution for
> changing the color of a hidden tab whose process gave output in the
> meantime:
>
> 1. Every tab has a unique ID that can be read and not modified. This ID is
> dubbed by the system at the creation of the tab.

This is pointless. Window configurations don't need a 'unique ID',
neither does this. Just reify the tab: make it a unique Lisp object type
(again, like window-configurations). Now the tab object serves as its
own unique ID.