I have about fifteen 9v wall warts within specs for the circuit; I'm going to use the one that fits the jack. There are three solder tabs on the jack. One for the positive tip, one for the negative ring/sleeve, and one for the chassis ground. My schematic has Vin and ground specified. I'm relying on the wall wart to appropriately ground the AC end of things. My pos/neg tabs are used for source voltage and ground, respectively, and that leave the third chassis ground tab.
Do I ignore it? Connect it to the negative side ground? To the chassis? What should I do when there's no specification about what to do with the extra tab?
I don't think you are correct about a "chassis ground".
Every barrel-connector jack like the image you provide has three pins - Tip, sleeve, and connector-present.
Basically, there is an internal switch that is opened by the presence of a plug in the connector. It's often used to disconnect internal batteries when the device is powered off of an external supply.
Short of a few rather unusual laptop power supplies, every barrel connector I have ever dealt with has been two-conductor only.