What is the proper way to isolate something to a project?

For example, suppose I wanted to make a notification list, sending email
to a set of people when a project changed.  I'd want that information
saved when I publish or export a project.

I suppose I could add my own instance variables and appropriate accessors
methods to Project, but I have a sneaking suspicion that wouldn't be saved
when exporting the project (because of Project's implementation of
objectForDataStream:).

I tried to isolate projects and use global variables, but that didn't seem
to work.

Any ideas?

-Eric