This is a great topic and thanks for the article.
I have to admit that this is way to difficult using the current toolset. However, if the semantics of the current flow model were slightly modified, it would become extremely simplified.
The issue is that the current flow model one works horizontally, i.e., ltr or rtl. In the case of direction:ltr, this really implies the flow of content is from left-to-right, top-to-bottom, as determined by overflow and wrap properties.
If the model was extended to allow for the CSS direction property to define top/bottom aspects of flow, then this problem would solved. To do this, there is needed additional values for direction to include: ttbltr, ttbrtl meaning Top to Bottom Left to Right, Top to Bottom Right to Left, respectively. The inverse of these may also be valid, but semantically, where is the bottom of the body block? I would advise staying away from that.
Back on point. Today, inline objects flow into lines that wrap at the boundary of the containing block. The inline objects can be characters, inline-blocks, etc. My site http://aviston.org is an example of how this is done.
In an enhanced model, a containing block could be defined to flow from ttbltr, i.e., top to bottom, right to left. The containing block would flow objects, not just glyphs, into the contained area using existing semantics for lines, margins, padding, etc. simply rotated to conform to the writing direction.
I realize this does not conform to the CSS3 model, but it would be great to have.