Real number symbol (UTF-8: ℝ) in XeTeX

How about this?

\documentclass{article}
\usepackage{amssymb}
\begin{document}
\begin{tabular}{ll}
Plain-\TeX{}:   & ${\rm I\!R}$\\
amssymb:        & $\mathbb{R}$
\end{tabular}
\end{document}

You should put your symbol format definitions in another TeX file; publications tend to have their own styles, and some may use bold Roman for fields like R instead of blackboard bold. You can swap nams.tex with aom.tex. I know, this is more common with LaTeX, but the principle still applies.

For example:

% paper.tex
\input nams.tex
$\realnumbers$ is connected.

% nams.tex
\def\realnumbers{\mathbb{R}}
% more definitions for the Notices.

% aom.tex
\def\realnumbers{\mathbf{R}}
% more definitions for the Annals.

Just change one line in paper.tex to submit to the Annals instead of the Notices.

With lualatex

\documentclass{article}
\usepackage{unicode-math}
\setmathfont{Latin Modern Math}
\begin{document}
$ℝ$
\end{document}

(not sure how to make it work with XeLaTeX or XeTeX)