I just got asked if I could do a Barbie cake with a sitting Barbie doll on it. This was the best example I could find on the internet, and I have to admit I think it looks super awkward. I kind of cringe thinking of making one of these as opposed to the endless creative possibilities a standing up Barbie dress cake lends itself to.
Looking at the design, I can't help but wonder how stable Barbie is sitting on the cake. I don't have a Barbie handy, but it looks like she has to lean back to sit. The weight of the fondant should help some to keep Barbie in position, but would she fall over backwards in transport and crack the dress?
From memory barbies flex at the hips enough to lean forward a little - could you use some supports under the dress behind her back that go into the cake and use royal icing to glue her to the fondant?
I think I'd make a kinda stiff wire wrap around her waist and to straight in to the cake, maybe even into the board. I'd do 2. Maybe you could tape a thin dowel rod onto her, front and back and cover it with the 'dress'. -- or popsicle stick up her back and into the cake?