I've seen this before. I can't remember what it was, I'll think on it awhile and get back to you. In the meantime, and it sounds strange, disable the power management feature in Bios. That's sticking out in my mind for some odd reason.
The other possibility that comes to mind is that the Bios might be seeing it as a keyboard, and getting freaked out when it doesn't respond as it expected. It would be normal for the computer to try to query keyboards at startup, and depending on the response generate a "Keyboard failure" message. It may be expecting some specific response or no response, and the IPAC might be returning some other value that's causing the Bios to hang due to a bug with handling an unexpected response. A Bios update could fix that.
But some odd reason, the power management is still sticking out in my mind as a cause for this. I swear someone had this problem a couple of years back and he and I tracked it back to that.