Tried it on DG4102 again, this time over USB, and the results are the same: no SCPI response, only an error message displayed on the DG4102 screen as it would be an unrecognized command, "Error generated by remote interface command!".
~$ echo "*IDN?" > /dev/usbtmc1; cat /dev/usbtmc1
Rigol Technologies,DG4102,DG4E17xxxxxx3,00.01.12
cat: /dev/usbtmc1: Connection timed out
~$ echo ":PROJ:STAT MCALTIMES,QUERY" > /dev/usbtmc1; cat /dev/usbtmc1
cat: /dev/usbtmc1: Connection timed out
~$ echo "*IDN?" > /dev/usbtmc1; cat /dev/usbtmc1
Rigol Technologies,DG4102,DG4E17xxxxxx3,00.01.12
cat: /dev/usbtmc1: Connection timed out
~$ echo ":PROJ:STAT MODEL,DG4162" > /dev/usbtmc1; cat /dev/usbtmc1
cat: /dev/usbtmc1: Connection timed out
~$ echo "*IDN?" > /dev/usbtmc1; cat /dev/usbtmc1
Rigol Technologies,DG4102,DG4E17xxxxxx3,00.01.12
cat: /dev/usbtmc1: Connection timed out
~$ #power cycled the DG4102 here
~$ echo "*IDN?" > /dev/usbtmc1; cat /dev/usbtmc1
Rigol Technologies,DG4102,DG4E17xxxxxx3,00.01.12
cat: /dev/usbtmc1: Connection timed out
~$
USB drive formatted FAT32, then copied only the 'keyfile.bin', plugged in the DG4102 at all times. When it was plugged in the first time, the USB drive was recognized just fine by the generator.