Is it possible to override the hardware home and sleep buttons on the iPhone? I did a lot of research on this, but I was unable to find any definitive answers out there.
I'm not looking to design something malicious, but I was curious about the possibility of doing this for a personal project on my own iPhone.
It is not possible, the operating system intercepts those interrupts in the kernel and dispatches messages appropriately.
This is possible on a jailbroken device only. There are addons available that in fact "catch" hardware button events. I can't tell you how it works, though.
I suggest looking for "jailbreak" and "home button" with a search engine of your choice, for a start.
Since this is for personal use, jailbreaking may be acceptable.
However on a non-jailbroken device (and certainly for any app that should hit the appstore) there is no way to intercept the hardware button events.