I was thinking the way that manufacturing mode is started up on an Xbox 360. You might be able to use that to run unsigned code. Cause from what it seems that it boots right before everything else, so I was thinking that we could be able to exploit it? I'm pretty sure what I'm thinking is probably not gonna work but, I'm thinking of something.
If its possible now one has discovered how / exploited it yet. Your best bet is the RGH hack now, it should work on the majority of consoles out at the moment.
Also I was looking at a few questions that were answered by one of the Fez developers and someone replied saying that the game would sometimes crash and reboot into a dos prompt? Could we possibly exploit this?
I know the old Peter Jacksons King Kong could be exploited from the shaders in a really old dashboard but that has since been fix. I believe the newest hack is RJtag, which I guess is a mixture of RGH and Jtag. Chances are if a method hasn't been found to run unsigned code without hardware, one won't be found.
I also heard that the game Fez will sometimes reboot into a DOS command prompt and stay there, maybe if this happens someone could use there keyboard or something and see if they could exploit it. (Can't have any patches on it or updates.)