I drive a Prius and have the clean fuel sticker, so I'm legal in there as a solo driver. But when they are specifically patrolling for that, they pull up and take a good look at me to be sure I have that sticker.

Early on, I had the sticker in the car but not on the window (due to being about to replace the windshield when I got the sticker), and I got pulled over and given a warning because it wasn't attached to the window. Based on that, I have to believe that they would pull a solo driver over just for violating the HOV lane (as a primary offense).