Do you think it would possible to make some sort of passive cap descorer that is triggered by hitting the field perimeter?
Possible? Yes. Practical? I don’t really think so. To make a passive descorer that works multiple times during a match, you’d need to convert the motion of running into the field perimeter (horizontal) to going up to the cap (vertical) Therefore, you’d need to somehow have a part either 6 or 14 inches long to trigger this vertical motion. Unless there’s another way of doing it, I don’t think this is practical.
Yes you could have a button like piece attached to a bar gear, attached to few gears to turn up the speed and final outcome length, and finally have those gears attached to another bar gear perpendicular to the ground. But it would need a lot of space or a lot of toque. Attached is a visual model from google
Untitled drawing.pdf (17.3 KB)
I think the word you’re looking for is a rack gear 
I love your mechanism though!
The big issue with this is the “trigger” for the descorer would be the robot contacting another object. So, if you got into a pushing match with another robot outside the expansion zone, your descorer could accidentally flip up. And if that happened repeatedly, it could be grounds for a DQ.
I think it makes more sense that you use the “bump” against the wall more as a power shifting device, so perhaps it pushes a few gears that changes a motor (for example, on intake) to run the descorer, that way you don’t run into the issues of potential violating the size constraints, or the issues of having enough power/torque to descore.