Just stepping back I have a question below. First let me say, please don't take offense if the question is too basic. I grapple all the time with the question of "is the model logic?", or "is it application specific logic?"
Remember that Magritte descriptions help define default dialogs/editors for *one* imagined application using the object, but there is no way you will define one set of Magritte descriptions for all applications of instances of your class. [At least for highly reusable model components.]
Even with "one" imagined solution, Magritte helps you put an interface on the object, but much of your interaction with the object may be through other code that does not stop to ask the MagritteDescription how to do something.
So my first question is: Is this a presentation problem/question or a model integrity problem/issue?
Why not define a method to get the #effectiveDate or whatever method. You said "to many" relationship, so which date should be displayed if there are 5 associated objects and they all have different dates? This sounds like some logic that belongs outside of the magritte description.
I quickly gave up trying to be "minimal" with Magritte descriptions. They are very handy and powerful. So I just clone them to create new ones that serve my needs... and move on.