By custom function codes, do you mean functions outside of 03, 06, etc. the standard MODBUS function set? Or are you having trouble within a particular function like reading a register value?
It may seem daunting but it is actually not particularly difficult to use the low level VISA functions to implement MODBUS calls. I did this for a project recently because I had a very limited number of functions I needed to use and I did not want to require other developers get the MODBUS toolkit. See attached example
