I found a replacement turn signal/brake light bulb at Kragen: the Sylvania 7528...same bulb as the 2008 Mercedes C-Class. The stock Phillips P21/5W converts directly to the Sylvania Long Life type...
Also interesting, the 451 smart bulb socket assembly does have three socket positions (as pictured above), but only the topmost and bottommost sockets are utilized. The center socket is empty.
Everything is working fine, and I learned all about smart tailights. Thanks everyone for the helpful input.