well, once again, do what feels best
actually, i have started playing open handed as well, thanks to you. this question is a good one
you can do either one, but i think it is easier to start the fill with the left hand, and come back to the hats on 1, like you said, but for me this is a bit difficult because i have to think about the fills i normally play out of reaction, the fills i know and dont have to think about, i have to sort of re-learn.
you could start the fills with your right hand and still come back on 1 with the left hand on the hats. you would have to be pretty quick, sometimes, though, with faster beats.
so my answer, like i said, is do what feels right... im no drum teacher, so i dont know if one way is "technically" correct, but if one of the two ways IS technically right, i would say it would be the first way, starting fills left-handed. while starting left handed is more difficult for me, when doing the fills i know second-nature, it feels like the right way to do them when playing open-handed.
hope this helps
butter