Oh my God They Banned Kenny
Again you've raised some interesting questions. In the past I have played that it can provide supply for 'the other side' if / when control changes hands. However, I think that raises a number of problems from a consistency perspective. Quoting the rule:
27.12 ... Tunis and Beirut are sources of supply for French units, but this supply cannot be shared with other nationalities. After these ports become Free French or Vichy they can supply only Free French or Vichy units respectively...
I think the part about "after they become" and "can supply only" could be read as it being a one time determination when French colony control is determined upon French surrender. Thinking about it, there isn't really any Free French control - FF units are in effect British units and would bestow British control on any enemy controlled hex they occupied. Vichy could not capture a hex while inactive and effectively becomes a German minor once active. So again there isn't really any possibility of Vichy gaining control per se (it would be German if Vichy units captured a hex). So I think the better way to read it is as a one time determination. If such a hex is captured by the other side it would be denied as a supply source, but it would not become one for the other side.