For the client devices to connect just masquerade will do.
However the problem here is to select access those devices from the VPN server or from the “ME on WWAN” point.
Different IP addresses will be needed to identify/separate the different clients, in a VPN server managed consistent IP network (with unique IP addresses and subnets). The translation to the local (and non-unique) IP addresses is then done before accessing them. e.g. The Client A and Client C subnet is mapped in a different subnet of the global consistent IP network.
Devices are addressed and connected with their global consistent IP address, that gets DST-NATted and routed to that local IP address.
The route may include the %interface, to make it unique for routing. But more can be done with connection marks and routing marks, routing tables, VRF, … .
http://forum.mikrotik.com/t/connecting-to-multiple-devices-with-same-ip-address/159052/1
http://forum.mikrotik.com/t/nat-device-to-connect-multiple-devices-devices-with-the-same-ip/138322/1