Best way to cut down on illegal usage would be with a FastPass type system - you get on without one - you get a ticket (failure to pay can result in non-renewal or suspension of registration) - great way for the state's to make money
No more so than already are used for any FastPass/SpeedPass violation now - rather than have cars posted along the road looking for offenders you already have them ticketed just as if they were using the toll roads without paying. Enforcement is already setup for that - you are just extending it to include any HOV lanes as well.
We actually have that in LA, it's called the 10/110 Metro Expresslanes. They replaced the previous HOV lanes and now require Fastrak transponders. No Fastrak means you get a ticket mailed to your license plate address. They have a switch on them which indicate if you have 1/2/3+ passengers. During rush hour on the 10 3+ is free, I believe 2 or 3+ is free the rest of the times for both, and 1 passenger simply pays a toll. However, that still doesn't prevent solo drivers from setting the switch to 2 or 3+ while alone and drive through the HOT lanes toll free. Enforcement is still required to ticket these drivers, but it's a little more difficult as they can use the excuse that they forget to set the switch correctly, which may or may not be an honest mistake. In the HOV lanes, it's a clear violation no matter what.