Range annotation between nothing and 100?

I guess you could override the Range object and add this behaviour.

public class OptionalRange : RangeAttribute {
    public override bool IsValid(object value) {
        if (value == null || (int)value == 0) return true;
        return base.IsValid(value);
    }
}

You shouldn't have to change the [Range] attribute, as [Range] and other built-in DataAnnotations validators no-op when given an empty value. Just make the property itself of type int? rather than int. Non-nullable ValueType properties (like int) are always automatically required.