For anyone stumbling in on this, please be patient. This is not in a playable state yet. We do have playtesters already.
The main netplay script is "\BizHawk-2.5.2\bbn3_netplay.lua"
Place your patched copy of "BBN3.gba" in the "\BizHawk-2.5.2\BBN3\ " folder. It must be the version compiled from the netcode branch.
There are two batch files for quickly launching the netplay script along with BBN3. To use "load double netplay.bat", make a copy of the "BBN3.gba" rom and rename it to "BBN3p2.gba". You may also modify the batch files to load any arbitrary rom names, or change how many instances are booted simultaneously.
According to Bizhawk's documentation, the scripting feature is only compatible with 64bit Windows systems.
There is nothing we can do about this. The Bizhawk developers simply haven't brought the scripting capabilities to Linux.
There are future netplay solutions in the works that will have improved platform accessibility, but they will take longer to deliver on.