I guess a tun/tap to some server-side service might be possible too, but perhaps not very interesting. The x86 dynamic library api suggested in the tech notes sounds more useful.

Edit: Easier idea might be adding ttyS1 and connecting that to a websocket on the server. Combined with something like socat or ppp, that ought to work. Since the console is already ttyS0 and connected to the js terminal, it might be doable even with the obfuscated source.

Or you can write a simple paravirtualized eth card driver and its implementation side, without trying to emulate real hardware. Linux is free software, take advantage of that.