I was originally thinking that the scarf in a post to make it taller would be best at the base of the post. I just thought that the top above any other joints may be where there is some stress that might not be the best location.
What would be wrong in putting a "post extension" on the bottom of the post and keeping the top in tact from top tenon down to post extension scarf?

Jim Rogers
PS. post scarf location maybe a question for an engineer to answer.