Actually, this has been done before, both with Four Swords Adventures and Final Fantasy: Crystal Chronicles (admittedly, the only two games that really work for Dolphin's GBA <--> GCN connection). You have to use a special version of VBA-M; have a look here: https://forums.dolphin-emu.org/Thread-gba-gcn-connection-guide. The instance of VBA-M can be running on any networked computer, as long as all of the instances of VBA-M can connect to the computer running Dolphin. The key is to change the IP address when messing around with Joybus options in VBA-M.
EDIT: Nvm, forgot that you can't see the Dolphin screen :p You could still netplay this way, you just couldn't see what was happening in the game.
EDIT AGAIN: On second thought, I suppose you could have people connect to the hosting instance of Dolphin with VBA-M, but also connect to that same instance with Dolphin's netplay spectators. This is a very convoluted and round-about way of doing things, but it should work in theory. I'm not an expert with Dolphin's netplay though, so I may just be doing wishful thinking after all.
EDIT: Nvm, forgot that you can't see the Dolphin screen :p You could still netplay this way, you just couldn't see what was happening in the game.
EDIT AGAIN: On second thought, I suppose you could have people connect to the hosting instance of Dolphin with VBA-M, but also connect to that same instance with Dolphin's netplay spectators. This is a very convoluted and round-about way of doing things, but it should work in theory. I'm not an expert with Dolphin's netplay though, so I may just be doing wishful thinking after all.