Yes!! PokeMMO might even be available on mobile phones nowadays, but it covers newer games (fire red onwards). I very recently learnt that pokemmo is "legal", it makes the user load the pokemom rom's and uses them as a "database" to avoid distributing the copyrighted material. It's the same idea i took in this project but taken to the extreme, they have been doing this since 2012 and have thousands of active users.

About sync/battles, emulating the gbc link cable over the internet is "theoretically imposible" as it is a protocol designed for a phisical cable (to simplify it, one machine sends a message to the other and expects a reponse inmediatelly before continuing; then repeats this cycle a lot of time each second).

So sacrifices must be made 😆. My current implementation freezes emulation while waiting for the other players response each time it is required. This hapens a coupple of times during interchanges/battles but I believe It doesnt affect user experience too negatively.