If she wants a classic Napoleon with the soft filling amongst the crispy dough sheets, I don't see how you can frost it with anything other than the traditional glaze on top. It doesn't seem to be a cake type or
construction that will hold up under much weight of icing or fondant.
You could probably construct/shape the cake as a cross, since it's usually a rectangle, anyway.
Just my .02
Rae
indydebi.. i think you're thinking neopolitan.
napoleon : 
neopolitan :
if the first is what your customer wants, you might be able to put some small RI flowers, like sweetpeas or rosebuds but you wouldn't want to use too many or anything heavier like bc, as it might be too heavy.
if the 2nd is what she's wanting.. go with indydebi's suggestions of 3 - 1 inch layers of the 3 differ flavors.
HTH
im going to as a stupid question - are you talking about a "napoleon" cake that is crispy pasry and cream or are you talking about "neopolitan" cake that is actual cake that is chocolate, vanilla and strawberry like ice cream. just want to be sure on what you meant
you can cover it with whipped cream ( i sweeten it with powdered sugar)
you can also make flower decorations with the whipped cream. The cake will not sit if you refrigerate it for a while before decorating it. whipped cream actually makes the napoleon taste pretty good.
I wonder if you could do something like this with strained preserves set up with gelatin or something. I don't know why this seems to be so napolean-ish to me. I don't know, stained glass just came to mind for this. You could do the design on each one and have them just all set together to make the cross? Or you could something with poured sugar shards.
http://www.macdonaldglass.com/products.cgi?catid=2&prodid=41
http://www.macdonaldglass.com/products.cgi?catid=2&prodid=32
Just an idea to toss in the idea pot.
DUH! You're right!! I was! Ooops!! 
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