I don't know that the HMI will allow Ignition to access tags defined on it. In our similar use case we did a one way PLC1 -> HMI -> PLC2 -> Ignition transfer of data because PLC1 didn't have ethernet comms (circa 1990 PLC) and we wanted monitoring in ignition.
In the C-More editor we defined several 'Tag Copy' events to move the data from the HMI to the second PLC's tags in the HMI. If you are looking for monitoring only then this approach while having Ignition behave as a modbus server will probably work.
The fastest rate these tag copy events run is once per minute. You can technically run a tag change event off the internal HMI system tags (250ms, 500ms and 1s) and define tag copy actions there, but I would be careful of the load on the HMI processor.