Don..........
At the ASA National School I attended, the instructors made it perfectly clear that it was OK for lefties to make the "sell" overhand out call lefthanded.
Their reasoning was the same that you stated. One of the instructors was left handed............and unless you were looking for it.....it was not noticable.
Relax.......the only people who would even notice would be another umpire........and ASA says it's OK!
Give it some practice.......it's got to be more natural.
I tried to swing a golf club left-handed once..........Ugliest thing you ever saw....
Joel