Typesetting linguistic rewrite rule

Here's a version that looks ok. I added a \twoof command for the second brace and made a longer \phold command \Phold. Since the underlying commands are tabular you can pass any tabular specification to it so I removed the intercolumn space to make the alignment look a bit nicer.

\documentclass{article}
\usepackage{phonrule}
\usepackage{amsmath}
\newcommand*{\twoof}[2][c]{%
  \ensuremath{%
    \left\{
    \begin{tabular}{#1#1}#2\end{tabular}
    \right\}
  }%
}
\newcommand*{\Phold}{\rule[-4pt]{2.5em}{.5pt}}

\begin{document}

\phonc
    {\phonfeat{$-$cnt\\ +hi}}
    {[$\alpha$grv]}
    {\twoof[@{}c@{}]{%
        \phonfeat{$-$cnt\\ $\alpha$grv} & \Phold\\
        \Phold & \phonfeat{$-$cnt\\ $\alpha$vce}%
    }}

\end{document}

You can look at the reimplementation of phonrule I did for Alignment in \oneof from the phonrule package

We can now define a variant of \oneof where the closing brace is added and the space between columns is removed.

\documentclass{article}

% BEGIN reimplementation of phonrule
\providecommand*{\textrightarrow}{\ensuremath{\rightarrow}}
\providecommand*{\textplus}{\ensuremath{+}}
\providecommand*{\textminus}{\ensuremath{-}}

\newcommand*{\phold}{\rule[-1.5pt]{1.5ex}{.5pt}}

\newcommand*{\phon}[2]{#1 \textrightarrow{} #2}
\newcommand*{\phonc}[3]{\phon{#1}{#2}~/~#3}
\newcommand*{\phonr}[3]{\phon{#1}{#2}~/~\phold#3}
\newcommand*{\phonl}[3]{\phon{#1}{#2}~/~#3\phold}
\newcommand*{\phonb}[4]{\phonl{#1}{#2}{#3}#4}

\makeatletter
\newcommand*{\env}{\@ifstar{\@env{&}}{\@env{~}}}
\newcommand*{\@env}[3]{#2~/#1#3}

\newcommand*{\envr}{\@ifstar{\@envr{&}}{\@envr{~}}}
\newcommand*{\@envr}[3]{\@env{#1}{#2}{\phold#3}}
\newcommand*{\envl}{\@ifstar{\@envl{&}}{\@envl{~}}}
\newcommand*{\@envl}[3]{\@env{#1}{#2}{#3\phold}}
\newcommand*{\envb}{\@ifstar{\@envb{&}}{\@envb{~}}}
\newcommand*{\@envb}[4]{\@env{#1}{#2}{#3\phold#4}}

% set environment for feature matrix with square brackets
\newcommand*{\phonfeat}[1]{%
  \ensuremath{%
    \left[\begin{tabular}{c}#1\end{tabular}\right]%
  }%
}
\newcommand*{\oneof}[2][c]{%
  \ensuremath{%
    \left\{
    \begin{tabular}{#1#1}#2\end{tabular}
    \right.
  }%
}
\makeatother
% END

\newcommand{\oneofvar}[2][c]{%
  \ensuremath{%
    \left\{
    \begin{tabular}{#1@{}#1}#2\end{tabular}
    \right\}
  }%
}

\begin{document}

\phonc
    {\phonfeat{$-$cnt\\ +hi}}
    {[$\alpha$grv]}
    {\oneofvar{%
        \phonfeat{$-$cnt\\ $\alpha$grv} & \hrulefill \\
        \hrulefill & \phonfeat{$-$cnt\\ $\alpha$vce}%
    }}

\end{document}