With FileMaker Advanced, you can install a custom menu that does not have the Show All Records and Show Omitted Only options that can defeat your filtering. When you use a custom menu to remove an item from the menu, you also disable its keyboard shortcut.
You can also create scripts for Show All and Show Omitted Only that replace the standard action in the menu and which Show All or Show Omitted, but then filters down to just the records matching the ID in the variable.
You can also use the OnModeExit Trigger to constrain the found set whenever the user enters browse mode. This can allow the user to perform finds, but the system then automatically filters out the records that don't match the currently specified ID number.
You can also use Manage | Security settings to limit the records that are accessible to the ID specified in the global variable. In this case, finds performed by the user will automatically filter to show just the permitted records. Show All and Show Omitted will, unless you modify the menu, still show record not matching the specified ID number, but they will be covered by "access denied" shields keeping the user from accessing that specific record.
Thank you. So it is more or less like I thought it would have to be done.
I hoped there would be a build in function like prefilter_results(field: value);
Well, seems it is time to go into scripting.