\newboolean{@plainText}
\setboolean{@plainText}{false}
\def\enablePlainText{\setboolean{@plainText}{true}\directlua{plainText=true}}
\def\disablePlainText{\setboolean{@plainText}{false}\directlua{plainText=false}}
\newcommand{\ifPlainText}[2]{\if@plainText #1\else#2\fi}

\let\@textbf=\textbf\renewcommand{\textbf}[1]{\ifPlainText{**#1**}{\@textbf{#1}}}
\let\@textit=\textit\renewcommand{\textit}[1]{\ifPlainText{*#1*}{\@textit{#1}}}
\let\@textsc=\textsc\renewcommand{\textsc}[1]{\ifPlainText{+#1+}{\@textsc{#1}}}