Single table, single record, one normal - not global - field, a script with a script parameter that sets its value to scriptparameter and commits the record. Run script with admin privileges, all other privileges have read only access to the field.
I'm not quite sure I get what you mean.
You mean a new table with a single record and field - I get that so far. But what do you mean with "sets its value to scriptparameter and commits the record"? I get what you mean with the rest (basically prevent anyone else from locking the record I believe by restricting privileges) but I'm not sure how you'd go about displaying the single record of a separate table on any other layout?
Script SetUniversalValue [content]
go to layout "UniversalValue"
set Field [Universal; get(ScriptParameter)]
go to layout [original layout]
you call it from another script, for example a script that displays a dialog box, gets user input in a global then calls the above script with the global's value as parameter.
for displaying the value on any layout, use a cartesian relationship to the UniversalValue TO.
if it's not obvious, I can build a demo.
The very important thing I didn't know about in your post, is the cartesian relationship - I was unaware that Filemaker let you do that. I changed the relationship between the tables and this automatically fixed my issue without needing any scripts - thanks for your help!