so i think now that the natural contact distribution on the coadjoint partial flag variety of an a-series simple lie algebra is given by the zariski tangent space of _either_ of the two schubert varieties "their point lies on our hyperplane" or "our point lies on their hyperplane", the two varieties being tangent to each other at the basepoint of the coadjoint partial flag variety. in the a2 and a3 cases it seems possible to visualize the tangency between the two schubert varieties in a number of ways. in the a2 case it seems almost obvious from staring at the weylotope and thinking about the bruhat genericity order on the schubert varieties corresponding to the vertexes, in that these schubert varieties are 2-d and each dominate both of the 1-d schubert varieties "their point is our point" and "their line is our line".

i'm not sure yet what to make of the general phenomenon of tangency between schubert varieties...

anyway, at least i seem to have resolved some of what was confusing me here; the "naturality" of the contact distribution seems to require the "self-duality" of the contact distribution, but i was skeptical about whether it really is self-dual. probably this self-duality is supposed to be conceptually intuitively obvious from a contact geometry viewpoint. it may be that the self-duality more or less amounts to "the envelope principle" stating that a hypersurface is generally the envelope of its tangent hyperplanes, where "envelope" is defined in terms of intersections of infinitesimally nearby tangent hyperplanes (or something like that). and of course the envelope principle is related to "legendre transform" which is all about the relevant kind of "duality". or something like that.