Modbus_send_custom_command - how does this work anyways?

Has anyone succesfully used modbus_send_custom_command()?
We need to implement modbus function code 16 which is not natively supported in the Polyscope modbus client.
First off - can anyone confirm the endian-ness of the data array?

The example in the script manual uses very short numbers so it is difficult to identify a pattern. How do I format the data array when writing to multiple registers with func 16?

For example:
I want to write dec 1 to register 404 and dec 10000 to 405:

start register 404 = 0x194
0x01 = 1 dec; 0x94 = 148

75535 dec = 0x0001270F
0x00 = 0 dec; 0x01 = 1 dec; 0x27 = 39 dec; 0x0F = 15 dec

resulting command:

this command doesnt seem to actually do anything at the modbus server, but it also doesnt return anything, so I dont know how to debug beyond just guess and check.