You can use the range
feature of unicode-math
to set the math font for a specific unicode slots or the macro accessing the glyph. I tried this with the sum sign & Latin Modern, see my example:
\documentclass{scrartcl}
\usepackage{amsmath}
\usepackage{unicode-math} % use unicode in math & setup math fonts
\setmathfont{Asana Math}
%\setmathfont[range=\sum]{Latin Modern Math} % substitute Sum sign
\begin{document}
Notice the different sum sign $\sum$, as intended.
Notice also, the difference in spacing, not intended:
\begin{equation}
\begin{bmatrix}
0 & \frac{1}{2} & \frac{2}{3} \\
\frac{3}{4} & 0 & \frac{4}{5} \\
\frac{5}{6} & \frac{6}{7} & 0
\end{bmatrix}
\begin{pmatrix}
A \\
B \\
C
\end{pmatrix}
\end{equation}
\end{document}
However, not only the sum sign, but also the spacing in the matrices changes. With Latin Modern commented out (as in the example):
With Latin Modern active (remove % before relevant line):
Why is that? BTW, I'm using
Lualatex
from TeXLive 2012.