Personally, I'd probably make it a "possession of the cell", item when 'k'ing over it (perhaps with a "to north/south/east/west" qualifier) and then visually either ignore it (like the "engravings off" setting does to those), give it to a highlighting background colour (non-black background compaitible with 'type of rock' foreground of the relevant railroad line symbols?) or (where the tileset allows) make a "wall/whatever with tapestry" picture (perhaps multiple, for various multiple wall designations, showing the correct side) which is essentially the railroad-line symbol with an additional highlight on the relevant side(s). Given that even the ANSI-like symbols are still pictures, I could see that being expanded in the 'default' charset, but I don't know all the technical issues involved. As with bridge-raising directions, the option of which side of the tile to place an item (where there's a choice) would be crucial in the build option. When the wall is removed (or another is built abutting to side) the tapestry would have to be either deconstructed or destroyed, as with other environmental (!!MAGMA!!) effects.
But I don't think most of those ideas quite solves all the problems that might occur. Until/unless we go isometric-tile or full 3D and can paint different sides of the walls with different patterns (
a la Sims or most FPSes that just work on internal rooms and ignores the relationship with walls on the 'other' side of a wall-gap), it does look to me to be a troublesome issue. Not saying this
isn't why it has happened, but I certainly wouldn't find it encouraging to not be able to find a good way of solving the issue, and might put it on the back-burner.
Of course, my thoughts probably don't match Toady's. Maybe he knows what he's going to do, when he gets onto this, or even knows that he doesn't want to do it anyway.