I LOVE your idea of practicing the bridle method at anchor. I will definitely try that next time I get a chance. I've been known to row out a stern or kedge anchor in order to avoid ferry wakes or other rollers. Your idea looks much better for making a rolly anchorage more comfortable and had the advantage of practicing a storm tactic to boot. Great suggestion!
I have always wondered the same thing about the Pardey Bridle. Seems to me like it would slide down against the hull. The solution I have in my head, is to depoly the sea anchor on a long rope rode (I have a 400footer I would use) that is attacked to my main anchor chain. An amount of anchor chain would be payed out from the boat. This would eliminate (mostly) the major issue of chafe on a rope rode. Also, having chain at the boat end of the sea anchor rode would allow you a fixed point to shackle (or otherwise secure) your bridle line.
Actually, since many sea anchor proponents recommend a length of chain at the boat end, I don't see why the main anchor chain isn't used more often. It seems like you could let out a few hundred feet of chain, rather than just a few feet, and it would aid in keeping the anchor sunk and allow you to adjust your wave/rode length as needed for your wave period.
"True, your boat will outperform mine to windward, but my boat will always outperform yours at anchor." --MedSailor