The MVFOI class has a function called getGeometry() that seems to do what I want, but I cannot see anything similar in the MVThemeBasedFOI class.
I thinks it's not possible for MVThemeBasedFOI. Nethertheless - when querying the MVThemeBasedFOIs FOIData (getFOIData) a geometry attribute would be very useful! I think its not included for performance reason. Maybe its possible to implement a functionality that returns the geometry of a single FOI within an MVThemeBasedFOI layer.