Yes, you can certainly use incident metering for portraits and other close subjects. Take a reading with the dome in front of the subject's face pointing back at the lens for the general exposure. If you want the contrast ratio, you can use a flat diffuser to take separate readings of the main and the fill (or to get a reading from the sun or the window providing the main light, and an ambient reading pointed away from a natural light source), or use your hand to shield the dome to get separate readings.

If you're shooting medium format or larger and using a handheld meter, don't forget to account for bellows factor at portrait distances.

It's also possible to use incident metering for distant subjects, as long as the light falling on the subject is the same as the light where you take the reading. BTZS works this way.