Sub-Project Sceduleing

To set up the scenario, my project is a sub to the main project contractor. For reasons known only to them, all I am allowed to have is the backed up sub-project to do my updates. It had never logically occured to me until recently that any external predessessor tie date disappears as soon as I do an F9 even with the "Recognize external relationships" checked. In other words, it is as if the tie does not exist and the internal tie becomes controling.

Does anyone know of any work arounds that don’t require plugging the dates each time I have to do an update which I am sure introduces a host of new problems