I'm in the process of doing this so don't actually have a photo yet, but I have made a bronze fitting which can be easily swapped out for a new one (I'm wary of future proofing my locomotive) which goes between the steam pipe from the dome and has a thread which my ball valve mounts onto. The allows me to mount the ball valve at the nose of the boiler in the right place. To enable me to drive the ball via the lever I intend to use a hollow shaft with a long screw through the middle with the same thread as the screw which currently holds the "domestic" lever on to the ball. With the correct slot in it this means I will be able to attach this shaft to the ball valve through the side of the smokebox. I haven't finalised the details but I suspect I will make a small plate with a closely fitting tube through which the shaft will go. This will allow me a little lattitude with alignment and make future changes of valve easier to accomodate.
This arrangement will not accomodate the full 90 degrees of travel of the ball valve but my initial experiments suggest that a full opening of the ball valve is very unlikely to be needed anyway. The ball valve will not be the greatest impediment to flow even at quite small openings, though it did seem to give a surprising amount of finesse on compressed air.