Our local supermarket was giving-out samples a few months ago. It was simple, but De-lish-us! I suspect that what they did could easily be adapted to the WSM.
First, I would pat it down with a good coating of Montreal Seasoning (or a simple mix of Kosher Salt, Coarse Cracked Pepper, Thyme, and Garlic).
They did theirs on a rotisseree, but I would think that running high-heat on the WSM, without a water pan, (meat on TOP rack) should work nicely. I would shoot for temperature of around 350°F, and cook to the proper internal temperature with a probe-type thermometer.
Low and slow probably not such a good idea for a good cut of meat like whole loin.
If you want to "Jazz it up", you could serve it with some sort of a tart fruit chutney - like currant or orange / cranberry.