For anyone stumbling in on this, please be patient. This is not in a playable state yet.
Place "bbn3_netplay.lua" in the "\BizHawk-2.5.2\ " folder when you're ready to test it.
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"