This is already possible to do.
For example, in AC21, the "Zone Stamp 02 21" Stamp can display any of the element Properties defined in the Property Manager, including your custom Properties.
In the Settings panel of the Zone Settings Dialog, select "Classification and Properties" as one of the items to be included in the "Content Order" page of the panel. Then in the "Classification and Properties" page of the panel you can select which Property should be listed.
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