I have never tried it (and not at a point where I could) have you tried nesting another iif? like =iif(Sum(iif(Fields!x.Value>0,Fields!x.Value,Nothing))=0,"-",Sum(iif(Fields!x.Value>0,Fields!x.Value,Nothing)) )
sorry - I am not at a place where I can confirm the format or the working - hopefully you get the idea...