Thanks for the anecdotes MLeake. I knew there were situations like yours where shoulder rigs were useful, but didn't have first hand experience with one. The closest I've come so far is sleeping wrong, and having a kink in my back.
As for the debate between you and Jammer Six, may I suggest a both-right-both-wrong compromise? From one angle, yes, being able to train using this carry method would be helpful. From the other, I wouldn't want to be around some stranger in my range I didn't know practicing his shoulder holster carry. I would have to re-read the webpage of my range's rules, but they don't allow any holster work at all, at least until you jump through hoops, ringing bells, and blowing whistles at a class/workshop/skillstest of some kind showing you can do so safely. At that point I believe they'll allow any holster work though I couldn't say 100%.
Even if someone did that, I would not want to be around anyone with a crossdraw or shoulder holster doing training at a range. To some extent it goes back to the same reason we all carry. I'm not going to trust Officer Smith, or anyone else, to be there, and ready when I need defending. Even if their heart is in the right place, it's just not possible. So you take care of preparing for that situation yourself. It's human nature that you trust yourself a hell of a lot more than some stranger. You know you'll be safe and careful, but that yutz in the next bay who obviously thinks he's Yosemite Sam with the strong-hip+Cross draw, or Denzel from Training Day...
My personal solution is to go bang at the range, and load up with snapcaps and do holster work at home, alternating mirror and eyes closed.