It depends on the device, but if it's a regular capacitance sensor, what's really needed is just some mildly conductive object touching the sensor - something akin to a human finger. Does not have to be personalized, since this is not a fingerprint sensor.

I have an older Yubikey stashed somewhere - if I can unearth it, I'll do some tests. But I suspect it could be something as simple as a wire: on one end touching the computer's ground circuit, while at the other coiled up to make a flat spiral the size of the sensor, and having that touch the sensor. The wire could probably be isolated, because the sensor is based on capacitance, so it does not require a full circuit.

A simple way to get a ground connection to the computer is via a fake USB connector. Pin #4 is ground.

http://www.hobbytronics.co.uk/usb-connector-pinout

Or perhaps the outer metal jacket of the USB connector would also get a ground connection.

With a Mac laptop, maybe it's enough to just touch the metallic case to get a ground connection.

TLDR: A wire, connected to computer's ground at one end, with a little metal pad (or flat coil) at the other. Bring the pad close enough to the sensor, and it should trigger.