But, I would make individual calls of the Modbus_master block, and assign a fixed address for the data for each call.
The Modbus_master block does not finish in one cycle. You have to monitor the DONE, BUSY, and ERROR bits, and control the REQ accordingly.
If you dynamically change the data, it becomes a mess.
Simply make an individual call of the Modbus_master block for the different data areas and with unique control bits.
This is something that gets a nightmare to debug if you try to be 'clever' just to save a bit of coding.