What with one thing and another, life got in the way of making any significant progress against the linking code until this weekend.
I finally got the time to put into plcd to finish up the linking code, and it appears to work in both directions.
plcd[1362]: PLMProtocol::LinkDeviceAsController: linking 1a1e8c01
plcd[1362]: PLMProtocol::GetNextLinkNumber: Next group is 2
plcd[1362]: PLMProtocol::LinkDeviceAsController: sent: 02640102
plcd[1362]: PLMProtocol::LinkDeviceAsController: received: 0264010206
plcd[1362]: PLMProtocol SS sent: 02621a1e8c0f0902