I've been approached to do a sculpted cake. The cake itself is a rather boxy design. The problem is that they want a black forest cake.  What can I use in place of whipped cream? Will the cake be ok with the whipped cream being non refrigerated for so long? 

No. Nor would the cherries. I would tell them that flavor can't be used for this design, as it needs refrigeration. Then offer another flavor that doesn't need refrigeration, or just a regular black forest cake for them to put in the fridge. 

Totally agree with @inthekitchen2. The closest I would get would be candied cherries with regular buttercream filling and chocolate cake layers. It is not worthy to risk beyond that... a collapse or even worst, a colective food poisoning!

People often think that bakers are magicians, there are things that are simply imposible....

I love the suggestion of a mini black forest cake to be enjoyed at home...