I think the only way you'd be able to run a sub in a zone 2/3 type of configuration would be using the speaker inputs and built-in crossover on a powered sub that has them.
This would allow you to send a full range stereo signal to zone 2/3, then you could route the L/R speaker wire through the sub's built-in crossover via the speaker inputs, then back out to your speakers via the speaker outputs, again, assuming the sub has these (several still do). This would allow you to set the crosover point on the sub to handle anything you determine and send the rest of the signal to the L/R speakers... TD
edit: example attached..