Howdy 1968rouleur,
Good question. It sounds to me like you might not even want a custom field at all, as those are usually unique for each event. Rather, you would just like to disaplay some extra text in the Meta area of each event. To do that you can use our themer’s guide. That will walk you through how to make changes to the template of your event, and one of the many changes you can do is add text to the meta box. Since you are changing the template, this will retroactively add that text for all past events, and add it to all future ones as well!
Does that make sense? Will that work for you? Please let me know. Cheers!
– Brook