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Idris,
A related point is that luatex is explicitly designed for extensibility of, e.g, primitives via lua, so there should also be a straightforward way for experts to implement the desired low-level behavior. For example, something along the lines of \pardirHR \pardirLR could be reimplemented in lua. That *might* be a better approach than the mirroring Javier requested and which Hans has agreed to do; especially if there is additional residue out there of more primitives that need this kind of treatment.
Actually, \parshape can be mirrored even in pure TeX. But I still believe this must be done at the primitive level, and not with some hack. I think luatex is about making even better paragraphs from good paragraphs, and not as a hack to fix some directional properties. Javier