An issue I've encountered with vPilot 2.0, is the restriction on callsigns, which insists you must start the callsign with a letter. This becomes an issue with some registrations. Take a look at the Israeli registration prefix: 4X. A friend of mine attempted to connect as 4XECA, but vPilot refused to connect because of the restriction.
Some examples for prefixes starting with a digit:
Congo - 9Q, Croatia - 9A, Cyprus - 5B, Jamaica - 6Y, and so on.
I understand you want users to connect with appropriate callsigns, but it shouldn't be an issue to connect with such prefixes.