I have not tried to use a variable voltage on the mixer diodes to control power through the unbalanced ring. Not sure if that would work, though would be curious as to the theory on how it would work.
You still need to adjust the CLOCK#2 signal of the Si5351 and turn off the other two CLOCKs.
To do as you are suggesting, you could install a switch between the CW-KEY and R104 to connect R104 to the CW-KEY or to a variable resistor to adjust the voltage that is connected between the CW-KEY and R104. Throw the switch and then press the CW key/PTT to get a lower level tune signal. Switch it back to do the normal operation. It would require that the CW mode be straight key, not keyer and that the CW mode is set in the v6 (not required in prior versions, just need the CW key connected).
Above are speculations, so I could be off.