Couple options here:
You could use netmap similar to this: http://forum.mikrotik.com/t/nat-1-1-mapping-using-netmap/97241/1
You’re just using the Public addresses routed to your service in place of local IPs.
Alternatively you could set your router up as the gateway of a DHCP server in the Public IP subnet (of which it already has an address) - then hand out Public IP leases to your devices directly instead of NATing. Bear in mind you will need to create an address list and exclude these from any src-nat or masquerade rules you may have in place at the gateway router.
Just brainstorming, let me know what you think.