Can I change the source address of a packet?

I assumed it was solved. One device with the same IP address and same IP subnet on all the independent ports, one device per port (ethernet ports).

http://forum.mikrotik.com/t/port-to-interfacing/97096/1

If not enough ethernet ports on the router , then maybe VLANs can be trunked to a multiport switch with access ports.
Well you will need one ethernet port per device. (Not solved for 100k devices, with one setup. But should work for some devices limited by # of ethernet ports)