Drilling a round hole is easy. Drilling an oval hole involves drilling two holes very close to each other. It sounds difficult. I can't imagine why anyone would want to make a pipe like this. The odds of destroying the briar blank are very high. Also, what would be the advantage, other than having a larger tobacco chamber? Just grab a larger drill bit. I don't see why the cake would build differently, if there was an oval bowl.
Now if you are talking about the outside of the pipe, then no problem. You can make the outside any shape you want. This would not have any affect on the thermodynamics of smoking or building cake.
Winton