I tried a couple of things today:
When you want to 'stand at ease' with your hands behind you or with your hands crossed in front of you, you'll notice that it's always your left hand that goes over your right hand and that's what feels comfortable. If you keep your right hand over the left, it just does not feel comfortable.
Same feeling when crossing the legs over each other in Seiza. When crossing your legs if you make contact with a large portion of your left foot over the right, it feels more comfortable than right over left, but it's tougher to distribute the weight of the upper body onto the legs and you'll end up crushing your feet a bit when sitting.
On the other hand, if you don't cross your toes while sitting, what it feels like, is similar to your upper body being like a wedge that is driven between the two feet (and the feet are driven away from each other sideways) and it feels loose and a bit uncomfortable to sit after a while (although it is reasonably comfortable once you get used to it).
When seated with a toe over the other toe, the feet get placed in a position that offers a kind of a locking mechanism that supports the weight of the upper body comfortably and since having the left toe over the right feels more comfortable, it is just a matter of positioning and comfort.
How this concept might have started:
Perhaps people in ancient times observed the phenomenon of feeling more comfortable having left over right, and tried to offer an explanation to the feeling by saying "left side is calmer than the right" etc. A Ukrainian who studies Hinduism in India heard the "calmer" philosophy and mentioned he's heard of a similar concept in India, where the left hand is considered to be like the moon (calm) and the right hand is like the sun (more vigour).
Now that we've fleshed out the details and heard some opinions from across the world, I believe that if a left handed person feels comfortable keeping their right toe over the left toe, they should be allowed to do so. The reason is that I believe we shouldn't follow tradition just because it is tradition, but we should follow it with an understanding of what it is meant for. That's the way we gain respect for our tradition. There are far too many cultures where people thrust ideas down the throats of people and use fear as a means of making people stick to tradition. It ruins the good intentions the tradition was started with, and leads to an eventual lack of knowledge and loss of the capability for scientific inquiry. The martial arts are applied, grown and adapted better when we understand it.