With the FN 45 I really do not think it is that it is "too new" as FN keeps changing their models and that one hasn't sold as weel as need to see aftermarket companies make oarts to "dangle"on the weapon.
I have not ever seen FO sights for that particular gun. A guy I shoot with yses a FN45 on occasion and he has a FO on it. I believe he had an adapter made or bought one so he could mount the sight to the adapter then the adapter to his FN45.
I apogize for not knowing more. He has said how hard it was for him to find something as simple as a FO sight for his gun also.