I did do a castor oil pack a couple of days prior to flush. then I had a colonic (day of the potion). I find the castor oil pack before a colonic results in an all around better session, for me.
The packs do open up the bile ducts, so you don't want to do one immediately before your flush start night (you want to have bile in there). But you can throw one on during your potion night when you want the ducts wide open and relaxed.
Regarding strictures, I don't know. The packs do soothe and lubricate the colon, so I imagine they can only help.