After five months, I just discovered a key lock that sits immediately behind the left rear passenger headrest. Turning the physical key in it unlocks the trunk. I assume the intent of the lock is if the smart entry remote fails, or the battery is dead. Unfortunately, it is completely out of view unless the seat back is tilted down, and you can't release the seat back unless the trunk is already open. As long as you are aware it's there, and you have an emergency, it is possible, but quite awkward to reach around the headrest and use the lock. I haven't found any mention of it in the Owners Manual, but I'm sure it's there, somewhere. I'm probably the only Clarity owner who didn't know the lock exists, but I thought it might be worthwhile to let anyone who doesn't, know about it.
It's mentioned briefly on page 564 of the Owners Manual, but so briefly they don't bother to tell you where the "cylinder" is!
If I didn't already know where it is, it might be a challenge to find it from that drawing. The lock is mechanically linked to the Emergency release inside the trunk. You can actually see it move through the window as you turn the key.
There aren't many car trunks with windows. Light coming through Clarity's trunk windows should make it easier to find the emergency release. However, the gangster who forced you into the trunk in the first place can use the inside window to keep an eye on you.