Unfortunately, NI currently does not offer a Modbus .NET library. It sounds like you have already looked into some of the recommended alternatives from the NI side of things as well. While I havent had a chance to use any specifically, I know there are quite a few 3rd party modbus libraries which you could probably use to get the program done in C# without the LVRTE. I would recommend that you post this request on either the Measurement Studio or Additional NI Software Idea exchange pages. Our R&D department frequently adds new features from popular suggestions. Best of luck!
Kyle M. Applications Engineering National Instruments