How about they monitor the lanes for single occupant vehicles with no right to be in the HOV lanes at all? That has been a problem ever since they opened the first HOV lanes, and nobody seems to care or do much enforcement.
In the San Diego area, before FasTrak, they found that about 15% of the cars in the HOV lane were solo drivers who were there illegally. After a year of FasTrak, where solo drivers could buy their way legally into the lane, the rate went down to about 3-3.5%. During this time the CHP stepped up enforcement, so that was another factor.
One reason for lax enforcement...and also for allowing the proverbial mother-and-baby in the HOV lane...is that the lane is generally under-utilized. So for all practical purposes (fuel efficiency, air pollution and congestion) it makes sense to allow some non-carpool cars from the standard lane to use the HOV lane.
Admittedly, most people wouldn't select "drivers who are willing to risk a $271 ticket" as the group most deserving of the lane, but
as a practical issue they're actually helping to increase fuel economy and reduce pollution and congestion. CHP is more interested in dealing with issues like speeding, reckless driving and drunk drivers than with non-safety-related issues.
As the HOV or HOT lane gets more crowded, it becomes more important to get these jerks out of the lane and make room for legitimate drivers. Giving them a legal alternative other than carpooling (which most people reject or can't do) is more effective than writing tickets; hence FasTrak and access for fuel efficient/low polluting cars.
On the bright side, for most people it's enough to know that people will think less of you if you cheat.
Jan