OpenType (just ignore file extension for the moment) is a rather dump standard in the sense that it requires the engine to have some knowledge about the writing system at hand. ... So, what we have here is that ConTeXt has no special knowledge about Indic scripts, and thus it will not apply the feature properly according the linguistic rules.
It seems to me that, since Hans can't be expected to write special code for dozens of Indic scripts, let alone for every script in the world, the pragmatic solution would be a method for Context to harness external engines (ICU, Pango, Graphite, Uniscribe, or whatever). If that's not possible, or if he doesn't want to do it, or until he is able to work with scholars on each special case, programs like Notepad will be able to do something that Context can't. As a casual user, it's not an urgent need for me. I had read that Aleph functionality had been integrated into LuaTeX (though I never found anything very detailed about that) and thought that everything was okay. I assume that Luatex/Context or some future TeX will have this functionality someday, however it's implemented. I'll just keep watching.