Mary Anne,
I have never tried this... Your idea of the toothpicks certainly sounds like it would work as long as the ganache is not hard... If it is, maybe you could warm a pin to make your hole so that it won't crack. What about melted chocolate?
Anyway, here is a Bump for you 
Let us know what works best, please... May just run across this one day.
Good luck,
Pat
Since the ganache does harden, you could use those flower picks that you use to put fresh flowers on a cake. Put them in your cascade patten before the ganache hardens, and then you've got predetermined places to put your flowers all ready for them. I'm not sure how to put them into the picks, having never used them before, but I'd think attching your gumpaste flowers to lollipop sticks would work fine. They're about the same thickness as many flower stems, and you can trim them short easily.
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