Have you considered using unsweetened, stabilized whipped cream in place of Buttercream? It is a real treat for anyone, and the lack of sugar is offset by the sweetness of the cake itself, or any fresh fruit fillings you may use as well.
Simply beat 8 oz softened, full-fat cream cheese with 1 tablespoon vanilla, and 1-2 teaspoons Truvia sugar substitute (if desired) until smooth. By pouring a thin stream, gradually beat in (on medium speed) 1 pint of heavy whipping cream. Continue beating until pioeabke consistency is reached. Pastel colors are best, as are simpler borders and decor. This smooths well, and is lovely without any fondant at all…
As per fondant- a diabetic had best just throw it away and enjoy his or her slim slice of cake!
K. Downer