I've used the Xl several times to join handrail pieces! Works great! I've always reference from the bottom side myself, because my rails were flat on the bottom. I would set the depth to about 40 mm and insert a nice 100 x 14 mm domino. Also I would add a Kreg pocket screw to the bottom side, making that joint tight and secure! You can plug the kreg hole if needed and it isn't to visible being on the bottom. It worked great for sweeps, turns, etc...
edit...(Sorry, just looking at the question again). I was thinking you were thinking of joining handrail sections together...I didn't look to closely at your queston, I was excited just thinking about joining handrail pieces...! [embarassed]
I nearly did a few years ago but decided to just make a simple jig and used a drill and dowels. The base was set in a rail so it was just the handrail that had to be dowelled.
You could make a similar jig for the domino or use the plastic jig festool sell for lining up narrow stuff.