As always, the trick is to remember the purpose of the HOV lane: keep traffic moving. That's part of the reason motorcycles are allowed. It's considered safer to let a motorcycle stay in motion than be in stop and go traffic [1]. And having been stuck in the Dallas MixMaster since the I-35E N HOV was shut down, I can say holding a motorcycle half clutched for half an hour is dang near painful. This is also the reason motorcycles are allowed to ride the line in traffic jams (also air-cooled engines need to stay in motion in summer).

Plus, if you don't have a passenger seat, the bike's at full occupancy, which ought to be at least near high occupancy!

[1] http://ops.fhwa.dot.gov/freewaymgmt/faq.htm#faq15

Edit: managed to get the highway wrong. Whoops.