very interesting and detailed :) I like the idea of putting the pick-up under the license plate! I've been putting piezos under the bridge of my oilcan guitars or just didn't put any pickups at all recently.

I've only ever used a regular pickup once on an oilcan guitar I made a few years ago and decided not too do it again because a) it didn't sound as nice when just played acoustically due to cutting into the top b) the pickup was in the way of my picking hand and c) I didn't get the oilcan sound that I was looking for, it sounded just like a regular electronic guitar when amplified

I might try to put a pickup under the oilcan in future though, thats a cool idea!
Question: Does the metal not interfere with the pickup in some way? Have to admit, my knowledge in woodworking is definitely stronger than in wiring, pickups & electronic stuff :P
Looking forward to hear a soundtest of the finished license plate guitar you're making!