An improved \rowstyle that takes the cell contents as an argument

\documentclass{article}
\usepackage{array}
\newsavebox\TBox
\newif\iffbox \fboxfalse
\newcommand\FB[1][true]{\global\csname fbox#1\endcsname}
\newcolumntype{C}{%
  >{\begin{lrbox}{\TBox}} 
  c 
  <{\end{lrbox}\iffbox\fbox{\usebox\TBox}\else\usebox\TBox\fi}}
\begin{document}
\begin{tabular}{*3C}
  One & Two & Three \\ \hline
  1 & 2 & 3 \\
  \fbox{4} & \fbox{5} & \fbox{6} \\
  4 & 5 & 6 \\\FB
  7 & 8 & 9 \\ \FB[false]
  1 & 2 & 3 
\end{tabular}
\end{document}

This is a hack into tabu code. I'm redefining the internal of \rowfont so that you can specify the equivalent of the > preamble construction and the equivalent of the < preamble construction (this one is new). Not sure it is fully robust.

\documentclass{standalone}
\usepackage{tabu}
\makeatletter
\renewcommand*\tabu@row@font[3][]{%
  \ifnum7=\currentgrouptype
    \global\let\tabu@@cellleft    \tabu@cellleft
    \global\let\tabu@@cellright   \tabu@cellright
    \global\let\tabu@@celllalign  \tabu@celllalign
    \global\let\tabu@@cellralign  \tabu@cellralign
    \global\let\tabu@@rowfontreset\tabu@rowfontreset
  \fi
  \global\let\tabu@rowfontreset \tabu@rowfont@reset
  \expandafter\gdef\expandafter\tabu@cellleft\expandafter{\tabu@cellleft #2}%
  \expandafter\gdef\expandafter\tabu@cellright\expandafter{\tabu@cellright #3}%
  \ifcsname tabu@cell@#1\endcsname       % row alignment
        \csname tabu@cell@#1\endcsname \fi
  \ifnum0=`{\fi}% end of group / noalign group
}% \rowfont
\begin{document}
\newsavebox{\mybox}
\begin{tabu}{ccc}
  One & Two & Three \\ \hline
  1 & 2 & 3 \\
  \rowfont{\begin{lrbox}{\mybox}}{\end{lrbox}\fbox{\usebox{\mybox}}} 4 & 5 & 6 \\
  7 & 8 & 9 \\ \hline
\end{tabu}
\end{document}