We used the following code fragment using the changepage package:
\newcommand{\raggedouter}{%
\checkoddpage%
\ifoddpage%
\RaggedRight%
\else%
\RaggedLeft%
\fi%
}
to provide a \raggedouter command, inspired by the tufte-latex class. Please see the following image to demonstrate the effect in a two-side book.

It is not uncommon and is done by Bringhurst's The Elements of Typographic Style or even in the German Der LaTeX-Begleiter by Mittelbach and Goosens. I would suspect the function in a package such as ragged2e. Does the upper code-fragment cover all possible cases? If there is a good solution, would it be an appropriate feature request for ragged2e?
\marginparper se: I use e.g.\DeclareCaptionJustification{raggedouter}{\raggedouter} \captionsetup{width=\marginparwidth, justification=raggedouter, font=footnotesize, skip = 0pt}with thecaptionpackage. – Andy Oct 21 '11 at 05:41\strictpagecheckas well to get this working. Perhaps that's obvious for a non-newbie(?), but may be worth mentioning. – Keelan Apr 02 '15 at 14:58