How to (selectively) hide margin notes?

Question

Consider the following LaTeX code, which contains an index entry my index.

\documentclass{scrartcl}
\usepackage{showidx}
\begin{document}
Hello, world!\index{myindex}
\end{document}

This typesets as follows, with the index keyword shown in the margin:

If the line \usepackage{showidx} is deleted or commented out:

\documentclass{scrartcl}
% \usepackage{showidx}
\begin{document}
Hello, world!\index{myindex}
\end{document}

the index keyword disappears from the margin:

A similar mechanism exists for labels using the showlabels package:

\documentclass{scrartcl}
\usepackage{showlabels}
\begin{document}
Hello, world!\label{mylabel}
\end{document}

This produces:

whereas deleting or commenting out the line \usepackage{showlabels} produces, as before:

Question

Is there a similar mechanism for margin notes? Consider, for example, the following piece of LaTeX code containing a margin note:

\documentclass{scrartcl}
\begin{document}
Hello, world!\marginpar{A margin note.}
\end{document}

This code typesets as follows:

Is it possible to easily hide all margin notes? More intricately, is it possible to assign identifiers to margin notes (in such a way that several notes can share a single identifier), and then to hide only those notes associated with certain identifiers?

A putative example

A putative piece of LaTeX code in which margin notes are assigned identifiers:

\documentclass{scrartcl}
\usepackage{lipsum}
\begin{document}
\lipsum[1][1-4]\marginpar[tempnote]{1st margin note}
\lipsum[2][1-4]\marginpar{2nd margin note}
\lipsum[3][1-4]\marginpar[tempnote]{3nd margin note}
\end{document}

This would produce:

Now the addition of the putative line \usepackage{hidemarginpar}[tempnote] to the preamble:

\documentclass{scrartcl}
\usepackage{hidemarginpar}[tempnote]
\usepackage{lipsum}
\begin{document}
\lipsum[1][1-4]\marginpar[tempnote]{1st margin note}
\lipsum[2][1-4]\marginpar{2nd margin note}
\lipsum[3][1-4]\marginpar[tempnote]{3nd margin note}
\end{document}

would produce:

Answer 1

Your idea of having something like

\usepackage{hidemarginpar}[tempnote]

is difficult to realize, because one should make \marginpar aware of this "label" (with all the risks coming from the redefinition of a kernel macro). It is better to have code to define your new margin notes from scratch.

\documentclass{article}
\makeatletter
\newcommand*{\newmarginstuff}[1]{%
   \expandafter@ifdefinable\csname @my@marg@#1\endcsname
      {\expandafter\let\csname @my@marg@#1\endcsname=\iftrue}%\fi
}
\newcommand*{\includemarginstuff}[1]{%
   @bsphack\expandafter\let\csname @my@marg@#1\endcsname=\iftrue@esphack
} %\fi
\newcommand*{\excludemarginstuff}[1]{%
   @bsphack\expandafter\let\csname @my@marg@#1\endcsname=\iffalse@esphack
} %\fi
\newcommand*{\marginstuff}[2][]{%
   \if\relax\detokenize{#1}\relax
      \expandafter@firstoftwo
   \else
      \expandafter@secondoftwo
   \fi
   {\marginpar{#2}}%
   {\csname @my@marg@#1\endcsname\marginpar{#2}\fi}%
}
\makeatother
\newmarginstuff{foo}
\newmarginstuff{baz}
\begin{document}
\parskip2ex
Text text text text text text text
\marginstuff[foo]{1st foo}
text text text text text text text text text
text text text text text text text text text
Text text text text text text text
\marginstuff{generic}
text text text text text text text text text
text text text text text text text text text
\excludemarginstuff{foo}
Text text text text text text text
\marginstuff[foo]{2nd foo}
text text text text text text text text text
text text text text text text text text text
Text text text text text text text
\marginstuff[baz]{1st baz}
text text text text text text text text text
text text text text text text text text text
\end{document}