Anyone know of a good OWB holster for a 4 3/4" SAA that rides pretty much horizontally with the bottom of the grip facing upward?

www.mernickleholsters.com/

Should have one for you, if not they do real good custom work.

This is a pretty poor example, but I saw something similar in passing while seeing the expendables movie while flipping through the channels. Sylvester Stallone wears a SAA in that manner

{Not trying to be flippant::}

SAA are supposed to be carried with your typical cowboy action belt.
http://www.epsaddlery.com/c-52-44-outfit.aspx

Unless you are wearing a trench coat, they are hard to conceal.

I wasn't looking for a concealed carry holster for it specifically. Just something to keep it accessible and out of the way at the same time. If riding a quad, horse, or motorcycle, on the belt behind me would keep it secured, protected, and out of the way while still being available. With a longer barrel a downward holster on the back would be very uncomfortable so I was looking for something horizontal. That being said, a jacket would actually conceal this type of holster well, if I were to locate such a beast.

Most cross draws are good for your intended use.  Some ride a bit higher than others, and if you are willing, try talking to a custom leathersmith.  

I have one that rides considerably higher than most.  The belt loops are almost half way down the length of the holster body.  This places it, on me just a bit lower than a shoulder holster.  The cant is a bit milder than most cross draws.  But then this one is for a specific set of concealment conditions.  Sadly the maker has passed.  But another really good one is Levergun Leatherworks

One thing I would suggest is that if you plan on riding quads or motorcycles get a full flap holster.  It's amazing the amount of crud that an open top holster can accumulate in a relatively short period of time during these pursuits.