I'm left-handed and years later I found I had written things on my grandma's (outside, stone) wall with chalk in mirror script. This dated from when I just started writing, so on an anecdotal level, this seems to be a natural way to write if you're left handed.
I started taking some of my whiteboard notes in mirror script after reading it assisted in neuroplasticity, and it really didn't take long to get the hang of. I haven't done it in a while though, can you still do it like muscle memory?