The only way I've successfully used a fountain pen for any length of time as a lefty was to write right to left with it. As with a right handed script you're drawing away from the finished script so you don't smudge it, but your angle and letters are "wrong" (until you get used to them that way). When I was in high school I read that Leanardo DaVinci wrote "backwards" in his notebooks[1] because it was more comfortable for him as a left handed writer, which of course as a nerd I had to try :-).

[1] http://legacy.mos.org/sln/Leonardo/LeonardoRighttoLeft.html