You probably want to try my alternate Modbus driver, and use multiple nodes in multiple instances, with each instance listening on a different port. You can have 255 nodes per instance, and every node/instance combination will have its own live storage. For efficiency, you should configure each node to only cover a limited range of addresses.
Consider running an extra Ignition gateway (in trial mode for dev, perhaps) just for such emulation tasks. Give the extra gateway multiple IP addresses if necessary.