% -*- coding: utf-8 -*-
\documentclass{article}

\usepackage{fontspec}

\begin{document}
\makeatletter
 \XeTeXinterchartokenstate=1
 \XeTeXcharclass `A=1
 \def\test{\fontspec{Arial}}

\XeTeXinterchartoks 0 1{\bgroup\test\get@external@font}

\XeTeXinterchartoks 1 0{\egroup}

\bfseries\itshape aAa

\bfseries\slshape aAa !!!

\XeTeXinterchartoks 0 1{\bgroup\test}
\bfseries\itshape aAa

\bfseries\slshape aAa

\end{document}

