Finally finished the new NES project I've been working on. A RAM replacement for the console that allows you read/write any memory location live via serial. You use your actual games in the console, they don't require any modification to run. The board also provides additional features like a remote reset / frame stepping for debugging.
Here is an example of the stuff you can do with it - pulling live data from Blades of Steel to create a scoreboard / trigger a relay on a goal.
https://vine.co/v/M6WzFBXeX1e
https://vine.co/v/M6WZlFjdJ69
http://www.batslyadams.com for technical details / more examples.
Also : yes it would make a great interface for a synth because of the bandwidth involved, it just wouldn't be very practical since you need to make a pretty major modification to the console. Having a cart is still the best way to go.