"You cannot provide accidental-damage insurance for a user adjusting their BIOS."

Rightly so because adjusting the BIOS won't cause harm!

PS: if you are referring to damage caused by oveclocking (if perchance it's available in the BIOS), then this is a user-accessible feature. As such, it'd be covered under warranty.

If a manufacturer played hardball and tried to dishonor the warranty then they wouldn't stand a chance against most consumer legislation in most parts of the world. They'd be toast where I am, not only would they have to honor the warranty but they'd be fined in the process.

Perhaps you're in a part of the US where consumer legislation is essentially nonexistent then things might be different. (The US is known worldwide for having the worst consumer legislation in the Western world.)