You would need a lot of IT knowledge to pull it off, but since there was no official announcement, you will most likely need a time machine 😅. No one expected the servers to go offline so nobody thought of sniffing the packets with the intent of bringing the back.
The servers for Rock Band Blitz are still online so I don't see a reason why The Beatles' and RB2's servers should be offline. You should try and pressure HMX into giving an official statement on the server status.
Alternatively you could hope that HMX releases the source code for the servers so we can start making them ourselves, but that's even more unlikely I'm affraid.