Oso, thanks again for you kind compliments
My message tonight is: Take courage my friend, I have not seen lefties have problems with any aspect of life - why should calligraphy be an exception? I have been thinking about this for a while (since a left hander asked me a question in the Pilot thread). I was hesitant there and I should not be before giving it some thought. So here is what I came up with - a method to use flex nibs by lefties.

The use of a flex nib by either a left or a right hander has two crucial aspects.
1)
The need for pressure control . It is actually obvious but we forget it often that normal writing includes many ascending and decending lines. Whether you are a left or a righ-handed there are as many aasccenders as decenders. Pressure control is absolutely necessary in both cases. If you have great, if not then the first task it to acquire it. As James menioned above - the key issue is to master the light touch. In the old times - this was the way that people wrote - maybe many nibs were scratchy so they had to

Today everybody can press down (trained by the ball points), but modern users have difficulty to move the pen while it just floats above the paper. This is the motion that creates the thin stroke and is the one along which the pen may dig into the paper. Trust me
even right handers" have a problem if they can not master the light touch.
2)
The need for proper position of the nib with respect to the slope of the letters and the requirement to press ONLY on the "downstrokes" , i.e. when the nib moves "backwards" with respect to the tip. The "best" orientation of the nib is along the slope of the letters. Now the trick in using flex nibs is to press only on the “downstroke”. I put quotes around downstroke because for I am referring to the motion of the pen away from the point it touches the paper and this will be “downwards” for a underwriter and “upwards” for an overwriter.
Therefore the “secret” for good flex is to understand the relative position of the pen with respect to the line. The “classic” (right-handed) calligraphy has an optimized motion of the pen for the formation of each letter. If an overwriter follows the classic sense of letter formation the result is odd. In this case the thick line forms in the "wrong" side of the letter.

So let's think about the possible positions. There are four - two for overwriters and two for underwriters. The classic calligraphy position is the one shown in the bottom left of the diagram below (see also
discussion here) If the natural position of the hand brings the pen to another angle, you can always correct it by rotating the paper. Or if a reverse slope is acceptable you can use the position in the lower right of the diagram.
Actually if we think about it, it does not matter whether someone is left or right hander. What matters is, whether you are an overwriter or an underwriter. The problem there for is for the overwriters. My idea

is that the overwriters can produce a nice script by forming the letters in the opposite way than the classic one. See top left and right sketch below in the formation of letter "b"

Does it work? Of course. I tried it myself - with my left and right hand in the overwriter's position (top and bottom in the sketch below respectively).

The only problem is that there is a large gap between the end of a letter and the beginning of the next one. The classic letter formation is optimized to minimize this gap. I believe people could experiment and come up with now letter forms

So there you have it. I hope it helps.