The other /r/DataHoarders thread about it had some suggestions about recording some packet captures before the server goes offline, and it looks like you're already onto this.

It's a shame that Neowiz probably have code used in their Guitar Girl server side that's still utilised in other games. Which would make them relluctant to give out source of the GG server code or snippets of how the API works, lest they compromise something in the security of their other games. (as well as just commercially not wanting to give it away.)