I've thought about how hard it must be to be a lefty fountain pen user. What is one to do? Learn to write without resting your hand on the page?
Actually, paper orientation was a big help too. Used to tilt the sheet so you write almost bottom-to-top, that way you at least don't smuge the writing right away, and hopefully the ink dries quick enough for the next line
I think I would write mirrored. Especially in this day and age, there is no reason to write LTR over RTL by hand. You basically never have to write legibly for official reasons.
If you read the linked post, he covers this