[XeTeX] Fine tune selecting alternate glyphs

[Szabolcs Horvát]

Hello,

I'm a XeTeX beginner, so bear with me :)  It is possible to use 
alternate shapes for certain letters, if the font supports it, e.g. as 
in the following example:

\documentclass{article}
\usepackage{fontspec}
\usepackage{xunicode}
\usepackage{xltxtra}
\begin{document}
\fontspec{GFSNeohellenic-Regular}δζΞ

\fontspec[Alternate=0]{GFSNeohellenic-Regular}δζΞ
\end{document}

The font used here is GFS Neohellenic, downloadable freely from
http://www.greekfontsociety.gr/pages/en_typefaces20th.html

And now, the questions:

1. This method switches the letter shapes for all of δ, ζ, and Ξ.  Is it 
possible to switch only some of them?

2. Ξ has a third shape (as seen on the screenshot and the font's PDF 
brochure, in the link above).  How can I access it?  Alternate=1 (or any 
other number) did not work.

3. Is there any way to use the alternate β, θ and φ letter shapes from 
the font GFS Elpis (available from the same webpage)? 
\fontspec[Alternate=0]{...} will not enable them, even though they're 
listed as stylistic alternates in the font's PDF brochure.

Thank you for any replies in advance.