Darn - I thought Smithy's idea, particularly the tape suggestion, might be viable. 
But then I know nadda (still ) about chocolate, much less this kind of artistry.
That said ... if dental tape (or floss) would work as a food safe 'line tool' if one could figure out how to make it stable ... what about laying down/spraying in a very fine first layer and embedding the dental tape pushing it down so it is very close to what will be the surface eventually. The ends should be long enough so you can pull it off later after the bonbon is released from the mold. Would that rip the whole layer or worse, more, with it too? Or would it be feasible to paint a bit of glucose (or something else that is a teensy bit sticky) on the part of the tape that touches the mold so you can spray and it won't blow away.
