Two approaches are two different kind of design. One is L2 VLAN separated the segment and one is L2 w/ individual NM segment.
What I suggest is to combine both designs that use separated VLAN w/ loopback address that dedicated for the NM traffic. The reason is at L2, it can separated the NM traffic from the production traffic, and for L3, you easily to observe the NM host by different subnet of the loopback address.
However, if there is WAN link that cannot carry VLAN traffic, then you have to use second approach in the WAN link.
And, if it is a L2 switch only, then you have to create a NM VLAN and assoicate the loopback address to this VLAN for NM.
Pease feel free to comment and discuss.
Hope this help.