After beating myself over the head with this problem for several hours today, I've finally come to the conclusion that if you fail to name the .dtbo file correctly, capemgr will load it, but not create the DT nodes.
Works: LZ-P1-00A0.dtbo
Fails: P1-00A0.dtbo
Moreover, it appears to ignore the part-number and version specified in the root of the overlay, but rather takes it from the string catted to $SLOTS.
Is this the expected behavior? Seems less than ideal.