Hi - I think the Soy Garden would work as a substitute in terms of taste and texture, but the frosting is still going to be too high in fat to be safe for IBS. A powdered sugar glaze or seafoam icing is much safer.
I did just find a recipe for a chocolate frosting that looks nice and creamy in the picture, though I haven't tried it yet. It is:
1 1/2 c sifted powder sugar 3 T unsweetened cocoa powder 2 T soy milk 1 t butter substitute (try the Soy Garden) 1/2 t vanilla extract
Combine powdered sugar and cocoa in medium bowl and whisk well. Add soy milk, Soy Garden, and vanilla, whisking until smooth. Spread over cake.
Also, for chocolate cakes, simply dusting with powdered sugar and then sprinkling with fresh rose petals (or other fresh flowers) looks very beautiful and takes no time at all.