I wouldn't use a conditional format to do this. You'd need one setting to hide the contents of the field and another to reveal the layout text "By Other".
Use a calculation field:
If ( Registration Admin = "Local" ; "By Other" ; Total Invoices Sent )
Place this field on your layout instead of Total Invoices Sent.
I mostly understand what you mean about what I'd have to do to use Conditional Formatting for this although that lack of simplicity seems out of character with the rest of what I understand about FM 11 so far.
Nevertheless I have used your "fix" and found it quite satisfactory - thank you very much once again! (speaking of invoices, you should probably be sending me one!)