There is some documentation covering the current interface. This will allow you to get started, but once @jamesh and @zyga-snapd finish the improvements that James mentions it will change slightly, but the underlying idea will remain the same: you have a snap that can see content from other snaps.
About scripts, yes, this will be possible in the near future. @pstolowski has been working on landing interface hooks, which is exactly what you want in this case I think: executables that can run when the interface is connected and disconnected (before and after, on both ends).