Hi,
first of all I would like to wish you the best for your health! I hope your back will be ready to ride.
About the freecoaster: 9T is very common today, it is hard to find anything else. For KHE-type freecoasters (a lot of companies use the internals from KHE with their own hub shell) you might find a 11T driver, but more was never produced as far as I know.
All the hubs use one piece drivers today, so you can not change the sprocket but only the whole driver. I no driver with more teeth was produced, you do not have a chance. Maybe a good mechanic can weld a bigger sprocket onto one of these drivers, but I haven't seen this and I doubt that it can work properly (a welded sprocket that gets loose can cause a lot of pain by the way).
If you could find one of these
http://www.global-flat.com/parts/part/232it would be a solution, because here a removable threaded sprocket (min. 13T) was used. But the hub itself does not work as good as the modern ones (this one is more than 10 years old).
Maybe you can find a Taska hub. The older ones also had threaded sprockets as far as I know.
In general 48H is also hard to find nowadays (but they are cheap because of less demand).
Could you weld brake mounts on the seat stays to have more space for a small sprocket? Or do you also have to less space for the chain at the dropouts?
Good luck.