anything that you do with this seam will always show up in the finish. The only thing that i can think of, is to have someone come in to add acrylic to the seam and blend both sides together. I have seen manufacturers repairs their shower stalls/bathtubs that have been scratched, chiped or broken off. You can't even tell that anything had been done.
Now your issue will be to find a manufactureror dealer that will patch this when you didn't buy it from them.
I know one of the units that was repaired for me was made by Maax. Miroline is another that deals with acrylic tubs/stalls.
You can probably google both and you may be able to find a disributer close to you that will help you