If I told you that you could make 500 million dollars (after taxes) if you figured this out, suddenly you would be on top of this.
The stakes of Stuxnet was probably “the safety of the free world” in the eyes of the people who paid for the work
With each of those requirements, there are easier and faster ways to get data off a machine. If you can install software you can likely download data that software would access. If you can access the machine twice and install software that runs in the background between visits, you can install your keylogger/data collectors and simply record the data on the device.
Stuxnet wasn't about getting data out of the machine, it was about breaking machines. It worked well because it didn't require physical access to the machines. It spread as a virus through the regular updates that Iran was doing for their machines.
If you had such a stuxnet virus in your back pocket, then you can likely steal the data and record it back on the USB device (or any plugged in USB device) on the system.
Spy agencies have definitely used obscure data exfiltration techniques and they can afford spies.
It's a neat attack, but to pull it off you pretty much need both physical access AND the ability to install virused software on the target. Perhaps something the CIA/NSA pull off such as stuxnet? Even then, the point of an air-gapped computer is that installing such software would be pretty difficult in the first place.