Earlier quoted context omitted.
That would be a bad idea. GrapheneOS operates under the assumption that every attacker has encyclopedic knowledge of GOS at all times. This is by far not an unreasonable assumption. GrapheneOS does not add features relying on ignorance that they exist. The duress wipe feature shuts the device down to let RAM discharge, this is an important step to remove any components of decryption before they can be lifted.
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?
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.