Hi Mike,
Instead of using SPQ for due diligence, have you considered using d-forms. d-forms can have your own defined fields and go through a defined approval process as well. A SPQ followed by a dForm (tied to a review -> approval task) should help you achieve workflow for due diligence. Alternately you may look at customizing SPQ with visibility conditions provided you can pre-define your due diligence criterion based on a field that is being captured in some filed on SPQ.
Regards
Vilas