Ah, well, I personally don't see why you'd want this as we have a method of playing certain games through a tunnel.. Which isn't that complicated TBH... Just install the app, and make sure your console is hardwired in... This isn't like the Wii where it wasn't so easy.
Plus, modding your Xbox allows you access to the entire Xbox library..
The Xbox Classic never had downloadable game demos anyway. You have the Xbox Live arcade stuff which has all been preserved anyway so you can use this without Xbox Live, same with most of the DLC.
In all honesty you could learn how to play system link games over the internet long before an emulated Xbox Live happens. It would be nice to see though I admit. I don't think the demand is there with system link been so accessible.
Realistic to see it happen any time soon? No, there's really no incentive to work on it due to the vast majority of titles having system-link/LAN functionality.
This would be amazing if this ever happened. There are only so many games with system link and certain genres of games don't have system link at all like fighting games.