IMO, Romo will never throw a pass for a different team, he is done in that regard. But, if Dak does get injured and Dallas has a good record and/or heading to the playoffs.........Romo would listen.
It would take a special circumstance for him to try once more and only Dallas holds that card. Obviously, this would apply to the 2017 season, another year off from football is too much and he would probably lose that itch by 2018 after a year in the booth.
I do not see him going to any other team during the season and try to catch-up on the fly but he could jump right in for us if need be.
With that said, I think his playing career is over, on to the next chapter.