Interesting question. No such option is exposed, and I don't think it's doing anything clever with seat occupancy sensors. I'm not sure how aware it is of HOV restrictions in the first place. I think that for routes which are only restricted at certain times of day, it will freely use them, and for routes which are always HOV it will never use them.

There are also some HOT lanes around here (where you can either drive for free with 3+ people, or pay a toll with less) and it never routes me onto those, even though it happily routes me onto other toll roads. Not sure what's up with that.