Invariant in XML Dexterity model

Hi everyone,
is it possible to define an invariant in a dexterity xml model?

I have my model declared in the type xml profile as:
... <property name="model_file">my.dx.content:mytype.xml</property> ...

And I need to implement a invariant. I tried to write it down in the IMytype interface but it simply gets ignored.

Haven't found any doc about that. I found ho to declare a validator (in the xml model) but, you know, they are context unaware.