Stephen
Now I had a little go at trying to design a usable junkrig foro your mast.
The drawn mast is only 9.4m, with a little of it below the wl, as shown. I guess if you have two tubes, 5m long, this would let you join them with 60cm overlap.
On the shown rig I have reduced the battens from 4.9m (Ingeborg) to 4.6m. By picking a master sail with AR= 2.00 and scaling it to B=4.6m, the sail area ended up at 33sqm, giving an SA/Disp. = 18. This sail makes use of the 9.4m mast without pinching it.
Fore-aft position.
This is always tricky; in particular with long keel and integral rudder
I have put the mast as far forward as I dared. The actual position of the mast and sail depends on, among other things, how the boat balances with the original rig. If the tiller balance is next to neutral, then the JR may well be shifted aft about 20cm. If the BR gives some weather helm, then I suggest you fit the JR in the shown position.
Something like that...
Arne