From an IP point of view, one might want to category devices into the following roles
L2 Stub Nodes are the Lowpan "hosts", i.e., they don't forward traffic for other nodes.
If L2 Mesh-Forwarder Nodes are allowed to be battery-operated devices, the PAN will need to use beacons; otherwise these nodes would have to have their receivers enabled all the time.
L2 Stub Nodes could be RFDs, could be battery-operated, and a goal would be to make them work at a 1 % to 5 % duty cycle of the receiver circuitry. This probably requires a modified NeighborDiscovery Protocol.