@Vaporized Steel, @Plusck, @Capt. Hunt
Found another way to skin the cat.
I had trouble closing the stock fairings to my tanker structure, but it turns out an upside-down Procedural Fairings interstage adapter was able to do the trick quite nicely! Way less cost & mass, and none of the structural problems of mounting boosters to a fairing. Even flew this thing at timewarp without it blowing apart.
@fourfa Yes, I'm using it, but don't have the precision to get it it right down to 0. Even when fully zoomed in (using WASD Cam), when I have 4 Vernors in mirror symmetry around an empty X200-16 tank, the best I can get it to with pixel-level mouse movements is about 0.003 kNm. I'm a perfectionist for this particular craft and want it at exactly 0, hence am editing the CFG by hand. Was hoping someone knew from the mu or code or something exactly where the thrust point was.
By trial and error I found using a Y offset of 0.2009m zeroed out my torque (at least to the 3 decimals RCS Build Aid shows). e.g. If your tank is at Y = 15m (default for first part), place the Vernors at Y = 15.2009.