I don't use sub reports too often, but this is what I would try...
Create a couple of formulae, SubE and SubRest. SubRest would look something like:
shared booleanVar SubE := false;
""
SubE would have a true. In the Section Expert for the page footer click on the formula button for suppression:
Shared booleanVar SubE;
SubE = False
Hopefully this works or gives you some ideas of how to get to a solution that does.