My understanding is a dispute makes it basically my word against a camera, but I don't want to have to ID the person driving the car. Would some sort of 5th Amendment interpretation apply?
You have no obligation to assist the Prosecution in making their case. No part of the law requires you to exculpate yourself by inculpating another person. Even though the ticket invites you to "rat out" the driver, you cannot get in trouble for failing to do so. Elizabeth Powell
RCW 46.63.075 provides that the registered owner is presumed to be the driver of the vehicle. Subsection (2) provides that the registered owner may overcome this presumption only if he or she states, under oath, in a written statement to the court or in testimony before the court that the vehicle involved was, at the time, stolen or in the care, custody, or control of some person other than the registered owner.
Nothing in this language requires you to name anyone else. Doesn't mean that the court won't ask though. Stand your ground, and bring a copy of the statute with you (you can google it).