Afraid I can't add much to this.
You can find a discussion and comparison of the two materials at: http://www.differencebetween.net/object/difference-between-steel-and-titanium/
and at: https://answers.yahoo.com/question/index?qid=20100319171745AAOllge
. It sounds like in general, Titanium is likely to be stronger, but it will depend on what alloy of Titanium and what alloy of steel. Titanium is however more inert and therefore likely to give less problems running in Carbon especially in/around salt water.
I would of though that a 19mm titanium rod, being 7mm thicker than is what is normally used here, should be plenty strong enough. As long as you can get it and afford it. A few thoughts though. You need to put something through the rod to stop in turning in the blade. Drilling a titanium rod might be hard work?? Not sure, just a thought.
I am sure others may of tried it, but the only canoe I have seen with Titanium parts was the Orange IC that was brought over by Christian from OZ to UK for 2005 worlds. Called Austrasized (sp?) by Neil Witt, when he bought it, it was later called the 'flying Baked Bean' by Stu Budden that had it next. But I don't know whether the rudder post was Titanium on it, although I remember that other parts were.
We normally feel that 12mm stainless is strong enough, but I have known a few break, including my own, when almost out the Camel Estuary. When it went, it just snapped - no warning, just went, and I didn't like that, so when I built a new one, I really didn't want to use stainless again and built the post from carbon/kevlar. Since then, there have been many dire warnings about the inappropriateness of carbon rod for this use and I guess I would follow advice and not do it again, but on the other hand, it must be over 10 years now since I made it and other than resurfacing it with graphite/epoxy every year so, it appears to be doing fine.
No doubt in my mind that if I had some 19mm Titanium, I would feel more than confident in using it. Go for it and tell us how it goes....
What's the worst that can happen?