[Tex/LaTex] Bytefield: Word Group below blocks

Question

I would like to get an effect similar to bytefield's letf/right word groups on the bottom of a bytefield. I didn't see any obvious way to do this in the bytefield documentation. My Latex macro knowledge is not particularly strong, is there a good way to extend bytefield to support a bottom/top word group?

Exact request best explained by MWE:

\documentclass{article}
\usepackage[layout=letterpaper,margin=1in]{geometry}
\usepackage[endianness=big]{bytefield}
\bytefieldsetup{boxformatting={\centering\footnotesize}}
\begin{document}

A 32-bit piece of data is divided into a header and some subfields. The
header is further subdivided into two parts

\begin{center}
\begin{bytefield}[bitwidth=1.2em]{32}
  \bitheader{0-31} \\
  \bitbox{4}{Header.A}
  \bitbox{4}{Header.B}
  \bitbox{20}{Subfield 1}
  \bitbox{4}{Subfield 2}
\end{bytefield}
\end{center}

\noindent
Word groups are the right look / style, but only go on the left/right?

\begin{center}
\begin{bytefield}[bitwidth=1.2em]{32}
  \begin{leftwordgroup}{Header}
    \bitheader{0-31} \\
    \bitbox{4}{Header.A}
    \bitbox{4}{Header.B}
    \bitbox{20}{Subfield 1}
    \bitbox{4}{Subfield 2}
  \end{leftwordgroup}
\end{bytefield}
\end{center}

\noindent
I would like the brace / label below the whole header section, e.g.:
%
\begin{verbatim}

+-----------------------------------------
| Header.A | Header.B |      Subfield 1...
+-----------------------------------------
\_                  _/
  --------\/--------
        Header

\end{verbatim}

\end{document}

Answer 1

You can put the underbrace in a borderless \bitbox under the header.

In the example below we use amsmath for the \text command since \underbrace is a math construction:

\documentclass{article}
\usepackage[layout=letterpaper,margin=1in]{geometry}
\usepackage[endianness=big]{bytefield}
\bytefieldsetup{boxformatting={\centering\footnotesize}}
\usepackage{amsmath}
\begin{document}
\pagestyle{empty}
A 32-bit piece of data is divided into a header and some subfields. The
header is further subdivided into two parts

\begin{center}
\begin{bytefield}[bitwidth=1.2em]{32}
  \bitheader{0-31} \\
  \bitbox{4}{Header.A}
  \bitbox{4}{Header.B}
  \bitbox{20}{Subfield 1}
  \bitbox{4}{Subfield 2}\\
  \bitbox[t]{8}{$\underbrace{\hspace{11em}}_{\text{\normalsize Header}}$}
\end{bytefield}
\end{center}

\end{document}