Alexander,
If there is no pre-composed glyph xelatex _never_ assembles the glyph correctly.
This is just not true. The example I sent you works correctly on my machine; I deliberately included y-macron-acute which does not exist in precomposed form in Unicode.
I need to run to work right now but tonight I will try your other examples and send a screen shot. Meanwhile a very important question: are you on Mac, Windows, or Linux? On a Mac, if you use a font like Junicode, things probably won't work. Mac OS X defaults to using the native AAT font rendering system. In order to get OpenType positioning features to work, you have to tell XeTeX to use the ICU renderer. More later . . .
David
-------------------------------------------------- Subscriptions, Archive, and List information, etc.: http://tug.org/mailman/listinfo/xetex
