Yes! Motorcycles may use Metro ExpressLanes (FasTrak) for free without a transponder in Los Angeles.
For Free, No Transponder Needed in LA Express Lanes are open to motorcycles with standard motorcycle tags (in or out-of-state). However, “personalized plate, disabled plate or special category plate, you must register your vehicle with the Metro ExpressLanes” to avoid having to fight tickets. Contact Metro ExpressLanes.
However, for motorcycles in Orange County/Riverside County (The 91 Express Lanes) you MUST have a FasTrak transponder while traveling in designated entry lanes and then it’s free except Eastbound Mon-Fri 4-6pm when you must pay 50% of the toll. Orange County has some weird rules but there you go.
Remember the free usage only applies to LA County, other counties in California may have their own rules or guidelines.