I may be wrong, but I did not thing the concept of even/odd mode applied to rectangular waveguide. For a four port, even mode means you apply two in-phase signals (+ and +) to two inputs, and odd mode means you apply two out of phase signals (+ and -) to two inputs. Its just a mathematical concept that people use to calculate things like static capacitance/length to determine impedance. Two adjacent microstrips in "odd mode" will act like there is an "imaginary" ground plane right between them, for instance. If you put +1 volt on one strip's input, and -1 volt on the other strips input, there kind of has to be "0" volts right in the middle of the two strips.
In a waveguide there is no "voltage", because there is no center conductor or ground plane. So you can not say there is "zero voltage" at some point in the waveguide, because to even think of a voltage in a waveguide you have to integrate an Efield along a length. So you could not have an "odd mode" mathematical ground plane in the middle of a rectangular waveguide, because you are integrating over zero length (the thickness of the imaginary ground plane).
I know I am not saying this very clearly, but it my best try.