Hi, I opened an issue on the github page with similar proposal (including matchmaking): https://github.com/itchio/itch.io/issues/500
This would be an incredible feature. A documentation for such system wouldn't be so complicated if everything is returned as a JSON (or any other simple file to parse).
I imagine that each application (game) would need a App key to make such requests, too.
Requests could be done using any URL transfer library (of the game developer choice, take cURL as example), so the itch.io developers wouldn't need to create a client-side API, only the server side, that would provide informations such as who is currently playing the game (so matchmaking would be possible), which are their achievements and scores (in-game content).