Hi Kivet,
h**p://students.ee.sun.ac.za/~sjmarais/
With the link above, you will find they are using FEKO to model aperture-coupled microstrip antenna. This is the winner of FEKO student competition 2005. You may also get the source code from the webpage.
I've heard there is some other commerical code doing hybrid MoM and FEM, but I couldn't remember the name. As for FEKO, it is the so called FE-BI (finite element-boundary integral), which is quite often more efficient and easier to use, compared to the absorbing boundary condition (or PML) used in HFSS.