Get line width and inner sep of a TikZ node

Question

Is it possible to retrieve a node's inner sep and line width using the key tree? My exact use case is below.

I want to find the coordinates of the red marks in the image below, but not using the method I've used below. Basically I want what would be the north east anchor (or some other anchor on a rectangle) if it were to sit in the middle of the border line instead of the outer edge, as well as if it were to lie on the node's contents before inner sep is added. I don't want to use duplicate "phantom" nodes as I have below because in my use case the node's contents might be messy.

I also realize I can just save the line width and inner sep globally by using \pgfmathsetlength{}{}, applying them to the node and using the same macros to navigate relative to north east, or in similar fashion by defining custom keys. But I'd really like to ask the node directly, as though these values were specified as literals in the node's options like \node [inner sep=1.2em] {};.

Image:

Code:

\documentclass{standalone}
\usepackage{tikz}

\begin{document}
\begin{tikzpicture}

  \node [draw, line width=5pt, inner sep=8pt] (A) {foo};

  \node [draw=cyan, line width=2pt, inner sep=8pt] (B) {\phantom{foo}};
  \node [line width=0pt, inner sep=0pt] (C) {\phantom{foo}};
  \node [line width=0pt, inner sep=8pt] (D) {\phantom{foo}};

  % Basically I don't want to use nodes C and D, I want everything relative to A
  \draw [red] plot [mark=x] coordinates { (C.north east) };
  \draw [red] plot [mark=x] coordinates { (D.north east) };
  \draw [green] plot [mark=x] coordinates { (A.north east) };

\end{tikzpicture}
\end{document}