@Lee, perhaps I have been thinking about this incorrectly. Message length is but one of many fields — I used it only as an example. Suppose the protocol in question carries all sorts of information that must be extracted from bit fields within bytes (e.g., message length, message type, sensor ID, sensor values, etc.).
Consider how to build a domain that knows about this protocol and how to convert between messages packed into a byte stream and xtUML interface messages. Can you think of a way to do so without coupling the model compiler to the protocol?
In our case we would prefer to use MC-3020 as is.