Of course they could do that, or claim that the SSD somehow caused another failure. More likely, from what I've seen, is that someone that's not careful can rip a video connector off the Logic board, or something like that. We've all seen the pictures of this situation on this site.
It is safer to simply use a Thunderbolt case with a SSD in it, and boot off that. (or even a USB 3 case with SSD). I've run a few iMacs that way for a while, too. Very reliable, fast, and no tearing into the computer. I used a small rubber bungee cord to attach the case to the back of the stand on those.
As I've mentioned, a failed video card was replaced under warranty in an obviously upgraded iMac and they didn't blame the opening of the iMac on it's failure. Although, the video card issue was a known failure point at the time.