Hmmmm.. can't say I have ever tried that. Not sure if you will be able to make it stand up (if it is to be bigger than say a couple inches... it is a rather delicate flower and BC would never pipe that way (and stay). You might be able to make a small one though with a petal tip like a 104. You could easily do a BC transfer of a picture of a magnolia though. I say experiment with your rose tips.. maybe even the 127 would work with a stiff BC if you made it thick at the base.. but you might lose the delicate-ness of the flower.
Boy, that is a good question that I was just pondering myself this week. There is a bakery here where I live that always puts huge 4 to 6 inch wide magnolias on their cakes. And I know a lady who knows how to make them in buttercream and offered to show me. I need to ask her and learn how.
I did a search in the galleries and forums and found mostly gumpaste directions and pictures. Funny, there aren't any buttercream. I think you need a really big rose tip, the biggest they make, a good picture of a real magnolia and practice.
Good luck, I hope someone here can help. Being from MS myself, I'd love to learn to do this. It's our state flower 
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