Sorry to pester, but this
problem is barring me from moving my book project into ConTeXt this
weekend.
The use of
linenumbers seems to throw off the R2L mechanism. Example of the problem
attached.
The culprit is the function "boxed.stage_two" in page.lua which output
each text line in the line numbering
environment in left to right mode. When you have justified lines
everything is fine but when you have a ragged
line (e.g. the last line in a paragraph) ConTeXt aligns the context with
the left margin.