This summer when it got so hot where I live, I started making all of my filling dams out of cake ball mixture. It worked really well and no one complained about its presence or taste. You could use a white cake (I trim the crust off so it stays fairly white) and set the dam a half inch or so inside the cakes edge. Once the cake assembled, you could go back and pipe a rim of filling in the crevice left from placing the dam a little further in than you normally would. Or you could make a dam of white fondant.
Please also see:
http://cakecentral.com/cake-decorating-ftopict-731770-.html
HTH
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