But if the format of NRPN can be totally arbitrary, is it possible to read them? For example, we can easily read custom meta events since meta events have fixed format. But what if we encountered NRPN in a MIDI file? Is there reliable way to read it? Waiting for NRPN NULL (99/127 98/127)? But, for example, for RPN it's possible to omit NULL since data can be read by definition of RPN. So if we don't know NRPN, it's impossible to read it in general case?
Yes, of course we can't interpret NRPN data. But if format is arbitrary, we can't even read data. For example, we see following bytes sequence: 99/xx 98/yy 6/zz 6/rr. What is 6/rr? It's a new NRPN with data of rr, or 6/zz 6/rr together create two-byte value? With RPN we can easily understand where each new RPN starts and ends, even without NULL bytes, just by knowing format of each RPN.
Usually when both the Data MSB and the Data LSB are used, they are part of a single value, but again, with NRPN you cannot make that assumption.
Are you trying to decide how to present the data in a user interface? What is your goal?