Might the "bias toward richer drivers" be a red herring? After all, limiting the sticker availability to hybrids already presents a bias toward those who can afford a hybrid.

Since the carpool lane is largely an incentive to reduce the number of vehicles (directly reducing energy consumption, emissions and congestion - plus some of congestion's associated energy and emissions), and that incentive is reduced as more vehicles use the lane, selling permits to those who will seldom use them may be the desired outcome....

On the issue of co-domiciled occupants, I think there are some who carpool because the lane can be used. I know of one example, a couple with a long commute, whose work schedules are offset by two hours - but they save enough time by being able to use the carpool lane to make it workable and worthwhile.