That horizontal piece where your seat is attached, is that round? If so, what is the diameter? If it is a diameter that you could shim to install a 1 1/8 inch threadless high angle (~ 35 degree) stem, you could put a stem on it upwards and towards the rear. Then you could put a cut down handlebar or a one inch diameter wood dowel on it that so that it runs side to side. From that you could hang a Carradice style saddle bag.
I have a Carradice bag hanging from a stem on my road bike in the photo below, stem is shimmed to fit the seatpost, I used a painted wood dowel, put a stainless fender washer on each end so the bag straps do not slip off. In my case I put a DIY aluminum rod support under the bag but that is not necessarily needed if you have enough tire clearance below the bag.
Thinking outside the box, you can do just about anything you could possibly want to when it comes to bikes and how to hang a bag on them.