We have extensive documentation on the array interface, specifically how to extend getindex and setindex! for arbitrary array types. In 0.6, julia's broadcasting is more awesome and central than ever; it would seem appropriate to have some discussion about how one should go about specializing broadcasting behavior. An example of a package that's about to take a do-it-yourself plunge is JuliaImages/ImageMetadata.jl#14; if you follow that link, you'll see that there was at least one very cool surprise lurking for the unwary.