I figured it out. I added a reference to LeapXRServiceProvider script on my main camera, and I made LeapXRServiceProvider.cs's method transformFrame( ) into PUBLIC, not PROTECTED. Then, I get the Leap Frame/Hand data from Leap, which gives me units in Leap's coordinate system (millimeters), then I call transformFrame( ), which gives me a Frame/Hand data with Unity's coordinate system (meters, z flipped), now I can call HandModelBase's methods SetLeapHand( h ) and UpdateHand( ) with impunity. It all just works. Previously, that method was protected, and unavailable.
Let me know if this is the wrong approach, because it does seem to work for me and I don't know how to approach it differently. Thanks!