Rather than a field in a related table, I'd just use a field with global storage. Since such a field can be defined in any table and you can still access it from any layout, you can define it in any table that is convenient. I define a special "globals" table for such fields in most of my solutions.
Then a script trigger on this field can perform a find using the text in the global field to set up the needed find criteria to find all contacts of a specific type.
Here's a thread with a number of scripted find examples, that use data in global fields in this fashion, that may provide some useful ideas for you: Scripted Find Examples
That's exactly the type of info I was looking for.
I'm still working out how to do things in FM like that that are "no brainers" in more standard programming languages.