I'm understanding that the SMS has little physical rigidity to it, leading to part of the tacoing problem.
Maybe that contributes to the problem, but probably only to a very small extent, if any. The backplate portion of the Profile is no less rigid that the backplate on the outside of the Nomad rig. And, in fact, the BP-SMS Profile backplate is a bit wider (10" toward the top and 12" at the base) that the backplate on the Nomad (7" along the entire length), so it actually provides a greater 'deterence' to tacoing than the Nomad. The top 'flap' or fold of of the BP-SMS Profile wraps over the top of the wing, and attaches to the BP, holding at least the top of the wing in place. The bungees attach to the small rings toward the top of the SMS plate, and then lie across the top part of the lobes of the wing and keep them from tacoing too much. The problem is really with the bottoms of the lobes of the wing, which float up rather dramatically. (I never appreciated the extent until I loaned my BP/Rec Wing/Profile rig to an OW SM student and observed him U/W.) As the diver, you don't really notice it. You have to observe someone else.
So, what if you to took an ABS, or plastic, BP and sandwiched your wing between the two plates, got the DR beaver tail, or butt plate, which I believe easily attaches to a BP. You could easily attach bungees to the plastic plate. This would in essence fix some of the challenges of the SMS from OMS. It would be a matter of securing bottom corners of the wing so they don't taco.
While you could conceivably add an outer plate, it would not be necessary. I agree with Dive-aholic - it really isn't a good solution. Besides, you have already pointed out the real issue - 'It would be a matter of securing bottom corners of the wing so they don't taco'.
One of the things that CAN be done, depending on the type of wing, is to add tabs to the bottom corners of the wing. If you look at the Nomad wing, you see that what holds the wing bases in place is a rectangular 2" SS ring - attached to the wing with a short piece of 2" webbing sewn into the seam of the outer casing - that slips over the waist strap. It is a reasonable solution and I suspect that Lamar and the folks at DR came up with it to address the very issue we are discussing, the tendency of wings to taco when used with SM rigs. I had been thinking of doing that with my Rec Wing for several months, since I saw the taco with the student in October and appreciated how significiant it was. It should be straightforward (although not necessarily simple) to pull the bladder out of the case, evert the case (turn it inside out) and cut a small section of the stiching along the outside seam, insert a short loop of thin 2" webbing, with a SS rectangular ring on it, and resew the seam. Another option is to move the tips of the bladder lobes out of the way, burn a small circular hole through both sides of the tip of the case and put in a brass grommet, which can then be used to bungee the base of the wing to the BP waist strap. Possibly easier than stitching in a piece of webbing, even if not as 'elegant'.
One of the exciting aspects of SM is that it is still associated with a bit of a 'frontier' mentality among users, even though people have been diving SM for probably 50+ years. A lot of DIY is evident in SM rigs (Steve Bogaert's Razor being a great example, that became a commercial product). And, your questions reflect the fact that you are doing what many people pursuing SM do - looking for creative solutions to problems associated with a configuration where you don't have your tanks on your back. I don't think an extra plate is the way to go. But, finding a way to secure the bottom tips of the wing lobes to your waist strap just might be the answer.