What review have you seen, exactly? Because none of those are actually available to the public yet, the release date for those are in late September, so nobody knows it they could be hacked or not.
If you look at their Assembly video, you'll see everything is running from that black box, which also appears to control the LCD screen. It's likely a completely custom board, so until they actually release, we'll have no idea if this particular thing has any debug pads or USB ports or anything that can be used to modify what's running or not. At worst, you'd probably just have to replace the entire box entirely with a brand new LCD controller and a Raspberry Pi or something.
But generally speaking, the only one probably worth hacking would be the Street Fighter one since it has 6 buttons per player (so more functionality for games)...but even then, you're probably better off building your own arcade cabinet. This thing is made of cheap fiberboard, so it's going to have quality issues right off the bat, and for $300 (or $400, since the $300 price tag looks like a pre-order only thing), you could probably find a better quality blank cabinet that you can stuff a Raspi Pi and LCD monitor into for more game options and better construction.
