I2C via modbus registers

I have been able to get a T7 to send data to an I2C bus using lua scripting via Kipling 3.1.17's Lua Scipt Debugger.  However, I want to be able to do this via the T7's Modbus registers similar to, I think, what is discussed here.

The end goal is to have a PLC, over modbus, configure the T7's I2C settings and command the T7 to write/read to/from a remote I/O expander chip like a PCF8574.

Intermediately, I am trying to use the Register Matrix area in Kipling to directly write to those same modbus registers to imitate what I think I should be able to do from a PLC.

The problem I am having, is that I cannot get Kipling to allow me to change the I2C_DATA_TX register.  Every time I try, I get the following error: 
1. LJME Function Does Not Support This Type (1312)
  - LJM Error Name: LJME_FUNCTION_DOES_NOT_SUPPORT_THIS_TYPE
  - Description: The function does not support the given data type. For example, LJM_eReadName and LJM_eReadAddress do not support reading LJM_STRING values, which are too large.
  - Operation: write 
  - Arguments: address: "I2C_DATA_TX", value: null

Here is what some of the other I2C registers hold now:

I2C_SDA_DIONUM = 13     
I2C_SCL_DIONUM = 12
I2C_SPEED_THROTTLE = 0  
I2C_OPTIONS = 0        
I2C_SLAVE_ADDRESS =32

I2C_NUM_BYTES_TX = 1        
I2C_NUM_BYTES_RX = 0        
                
Any help would be greatly appreciated.