To answer my own question:
* traveling on 2/4
* made an appointment on 1/16 for 1/23 (plenty of appointments available)
* applied in person on 1/23
* picked up my passport and naturalization certificate (in the same envelope) on 1/25 at the Passport Agency
I had no problem scheduling an appointment at my local Passport Agency for an urgent upcoming trip using their online tool. There was actually plenty of availability.
The tool is at http://passportappointment.travel.state.gov
When applying in person for an urgent (<14 days) trip at an actual Passport Office, do they immediately return the original Certificate of Naturalization? Or will it be mailed back just as with an application through USPS?