I am wondering how much of the reported sternum soreness here is related to the bone itself, and how much is related to the ligaments and tendons in that area.
Once I knew my sternum had healed, I very slowly started doing push-ups again. These tend to strengthen a lot of muscles in the upper body, but also get that connective tissue limbered up and stretched. At 10 months post op, I have no sternal pain at all, and full feeling has returned to that area. I did have a mini-sternotomy which in my case meant that the upper 1/4 of the sternum was left intact, so this might be a factor as well.
I'm not suggesting that people start doing push-ups after surgery unless they've checked with their doctor first, since sometimes the sternum can take more than the normal 100 days to heal. But if it's OK with your doc, I think it's worth trying. I now do 3 sets of 15 pushups, with about a 20 seceond rest between sets. I started out by doing "half" pushups--only a few at a time, then slowly increased the number and the range of motion.
Good luck!
--John