Someone mentioned that there is such a thing as a juno sub osc mod that gives you a saw sub oscillator.
Now, I know nothing about this mod but I can immediately think of two ways of doing this:
1) Use my saw sub osc design that combines the saw with the square to shift and attenuate the saw wave and making a wave one octave down
2) Replicate the saw core - let the sub osc output control the reset transistor, and tap (a buffered version of) the charge voltage from the DAC output and attenuate it 50%
Should be an easy fix :)