I have seen some cakes done the way you describe.
Here's one:
http://www.cakecentral.com/modules.php?name=coppermine&file=displayimage&meta=allby&member_id=4989&cat=0&pos=1
Sure you can...depending on how large and heavy the flowers are, you can secure them to the sides with toothpicks, just make sure and tell the person that will be cutting the cake!
I guess I'm a slow poke! MelC beat me to hitting submit! Sorry!
Also royal icing might work too. I have used royal icing To hold stuff on the sides or my cakes.
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