What about a duress timer working as the reboot timer but it wipes if you don't unlock within the time period. Would that have any advantages for destruction of evidence or deniability?

That would not be viable because the hardware does not support it. It cannot be implemented in the OS because the OS can be turned off or exploited endlessly.

For GOS to consider it, it would likely need to be backed by the secure element.

Duress PIN is deemed acceptable to implement in the OS because it is expected that the user is the one to enter it, so it has not fallen into the hands of attackers who may bypass it. Once attackers have it, you are effectively gambling. Account for that in your threat model and do not let it get to that point.