There are closet rod hangers like these:
but I wouldn't really recommend any kind of free standing hardware. The problem is, no matter how you secure them, there will be a pretty significant amount of shear force on your jamb material. Most jambs are made out of inferior stock and will probably crack if you subject them to that kind of strain. The safest bet is to attach a full length support to either side like so:
This way the load is transferred down into the floor. In this example you have to remove the existing door stop and apply the support in the same position so that it acts as your stop. Run your screws into the space where the stop was, that way you if you should decide to remove your supports later on you can return your door to its original state with out having to fill a bunch of holes.