I don't know if this could be useful for you, but when I want to calculate a field inside a cavity in Eigenmode, I usually define a line inside the portion considered, and along the direction in which I want to see the field, and then evaluate the field along that line, by doing this:
Results -> Eigenmode parameters report -> rectangular plot
then in the Context part I select the geometry along which I want HFSS to calculate the field (in this case a defined line, but it could be a surface), and then Calculated Expressions -> Mag_E (if one wants electric field).
Of course you can also do this with calculator: this is useful and quick if you have to evaluate the field on a large number of defined geometries. Just make a script!