Just to understand a bit more : signs like equal are printed inside math environment with GFS Didot thanks to \definetypeface[myface][rm][Xserif][GFS Didot]. Since GFS Didot ,as a greek font has greek letters, why are these glyphs not used ? I suppose that when greek people are using directly alpha as a letter (not \alpha), it should take into account the GFS Didot font, shouldn' it ?
No time for complete answer, but you need to define the whole [math][GFS Didot] repertoaire first. This has no effect until you define one.
Someone needs to write "Unicode math support" for mkii, or you need to make your own virtual font.
See this file for example: http://source.contextgarden.net/math-lbr.tex in a similar way that would need to be done for general typefaces without math.
You might then be able to say something like \definetypeface[myface][mm][Xmath][GFS Didot], though I'm 100% sure that people would then start complaining very soon that they either: - want other math characters in their favorite font as well (like plus, minus, brackets, ...), not only Greek letters - complain that they don't get any symbol (no font has them all)
Mojca
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