Create a Subversion step to query for the necessary information (you would need to consult the SVN documentation on if/how that can be accomplished), and in the step's vbld_StepDone script event parse the output and fail the build if needed (Step.BuildStatus = vbldStepStatFailed).
http://www.visualbuild.com/Manual/scriptevents.htm
http://www.visualbuild.com/Manual/contintegsample.htm