[XeTeX] Single glyphs from a font

[Joel C. Salomon]

Folks,

I’m trying to create this effect:
    a² + b² = c²   (1)
    x² + y² = z²   (1′)
The “old” method is, of course:
    \begin{equation} \label{abc}
       a^2 + b^2 = c^2
    \end{equation}
    \begin{equation*} \label{abc:prime} \tag{\ref*{abc}${}^\prime$}
       x^2 + y^2 = z^2
    \end{equation*}
This works, but it pulls in the prime character from the CM Math font.

I tried to say
    \tag{\ref*{abc}′}
using the Unicode prime character [U+2032], but the text font I’m using
(Cambria) doesn’t have that glyph.  I’m using Cambria Math as well, so I
tried
    \tag{\ref*{abc}$′$}
but the character got ignored.

How can I get XeTeX to give me the single prime glyph from Cambria Math?
The documentation I’ve read implied I’d use fontspec and \namedglyph, or
else \XeTeXglyph, but I can’t see how to make it work.