I have an idea for implementing this into WMMA3, if possible via patch.
When you add a match with "No Division" assigned, a pop-up window could ask if you want it to be openweight or catchweight. If openweight, then the fighters come in at their ideal weights and duke it out -- basically for those fantasy fights between a lightweight and heavyweight. If catchweight, then the fighters will seek to find a middle ground they agree to fight at, which may be declined altogether.
Additionally, I would like to see the fight history list the weight as openweight or catchweight, instead of the numbers (192 or whatever they fight at).