Is there a way to define starred variants for a keyvalue system?

I don't know about PGF keys, but my idea would be to test whether the values are empty or not:

\documentclass{article}
\usepackage{xparse}

\NewDocumentCommand{\attrib}{m}{%
  \nopagebreak{\par\raggedleft\footnotesize #1\par}}

\ExplSyntaxOn
\keys_define:nn { ae/epigraph }
 {
  title .tl_set:N = \l_ae_epigraph_title_tl,
  title .initial:n = {},
  description .tl_set:N = \l_ae_epigraph_description_tl,
  description .initial:n = {},
  author .tl_set:N = \l_ae_epigraph_author_tl,
  author .initial:n = {},
  longestline .code:n = \ae_settowidth:n { #1 },
  afterskip .skip_set:N = \l_ae_epigraph_afterskip_skip,
  afterskip .initial:n = 1\baselineskip,
 }
\dim_new:N \l_ae_epigraph_longestline_dim
\dim_set:Nn \l_ae_epigraph_longestline_dim { 3in }

\cs_new_protected:Npn \ae_settowidth:n #1
 {
  \hbox_set:Nn \l_tmpa_box { #1 }
  \dim_set:Nn \l_ae_epigraph_longestline_dim { \box_wd:N \l_tmpa_box }
 }


\NewDocumentEnvironment{epigraph}{O{}}
  {
   \keys_set:nn { ae/epigraph } { #1 }
   \par\hspace*{\fill}
   \begin{minipage}[t]{\dim_eval:n { \l_ae_epigraph_longestline_dim+1em } }
  }
  {
   \par
   \group_begin:
   \raggedleft\footnotesize
   \tl_if_empty:NF \l_ae_epigraph_title_tl
    { \l_ae_epigraph_title_tl \\ }
   \tl_if_empty:NF \l_ae_epigraph_description_tl
    { \l_ae_epigraph_description_tl \\ }
   \tl_if_empty:NF \l_ae_epigraph_author_tl
    { \l_ae_epigraph_author_tl \\ }
   \vspace{-\prevdepth}
   \group_end:
   \vspace{1ex}
   \hrule
   \end{minipage}%
   \hspace*{\fill}%
   \vspace{\l_ae_epigraph_afterskip_skip}
  }
\ExplSyntaxOff

\pagestyle{empty}
\begin{document}

\begin{epigraph}[
  title={title},
  description={description},
  author={me},
  longestline=abcdefghijklm
]
  Scenario 1
\end{epigraph}

\begin{epigraph}[
  author={me},
  longestline=abcdefghijklm
]
  Scenario 2
\end{epigraph}

\begin{epigraph}[
  title={title},
  description={description},
  longestline=abcdefghijklm
]
  Scenario 3
\end{epigraph}

\begin{epigraph}[
  title={title},
  description={description},
  longestline=abcdefghijklm
]
  Scenario 4
\end{epigraph}

\begin{epigraph}[
  title={title},
  longestline=abcdefghijklm
]
  Scenario 5
\end{epigraph}

\begin{epigraph}[
  description={description},
  longestline=abcdefghijklm
]
  Scenario 6
\end{epigraph}

\end{document}

I'm using actual value-keys. These are setup with the .initial handler (every value has an initial value). This reduces overhead, in my opinion. As it just a wrapper for a \csname solution. A key of the path /tree/sub/name is stored in the macro \pgfk@/tree/sub/name. I don't see much of the reason why I'd define a key that simply sets another family of macros.

The macros starting with \@epi@… in the environment are merely temporarily macros since they are only defined in the environment (which groups its content).

Code

\documentclass{article}
\newcommand{\attrib}[1]{\nopagebreak{\par\raggedleft\footnotesize #1\par}}
%%----------
\makeatletter
%%----------
\newlength{\widthofwidestline}
\newlength{\afterEpigraphSkip}
\setlength{\widthofwidestline}{3in}
%%----------
\RequirePackage{pgfkeys}
\def\epigraphset{\pgfqkeys{/ae/my/epigraph/environment/set/options}}
\def\epigraphgetvalue#1{\pgfkeysgetvalue{/ae/my/epigraph/environment/set/options/#1}}
\epigraphset{
    title/.initial      =,
    description/.initial=,
    author/.initial     =,
    longest line/.code  = {\settowidth{\widthofwidestline}{#1}},
    afterskip/.code     = {\setlength{\afterEpigraphSkip}{#1}},
    afterskip=1\baselineskip
  }
\newenvironment{epigraph}[1]{
  \epigraphset{#1}%
  \par\hspace*{\fill}%
  \begin{minipage}[t]{\dimexpr\widthofwidestline+1em}%
}{
  \attrib{%
     \epigraphgetvalue{title}\@epi@title
     \epigraphgetvalue{description}\@epi@desc
     \epigraphgetvalue{author}\@epi@author
%     \noindent % needed (?) if you use \\
     \@epi@title
     \ifx\@epi@desc\pgfkeys@empty\else
       \par\@epi@desc
     \fi
     \ifx\@epi@author\pgfkeys@empty\else
       \par\@epi@author
     \fi
     \makebox[0pt][r]{\rule[-0.8pt]{1in}{0.4pt}}%
  }%
  \end{minipage}%
  \hspace*{\fill}%
  \vspace{\afterEpigraphSkip}%
}
\makeatother

\pagestyle{empty}
\begin{document}
\epigraphset{longest line=abcdefghijklm}
\begin{epigraph}{title=title,description=description,author=me}
  TDA
\end{epigraph}

\begin{epigraph}{author=me}
  \_\_A
\end{epigraph}
\begin{epigraph}{description=description}
  \_D\_
\end{epigraph}
\begin{epigraph}{title=title}
  T\_\_
\end{epigraph}

\begin{epigraph}{title=title,description=description}
  TD\_
\end{epigraph}
\begin{epigraph}{title=title,author=author}
  T\_A
\end{epigraph}
\begin{epigraph}{description=description,author=author}
  \_DA
\end{epigraph}

\end{document}

Output