Use b for the optional argument in the inner tabular:
\documentclass{memoir}
\newsubfloat{figure}
\begin{document}
\begin{figure}[H]
\centering%
\begin{tabular}{@{}lr@{}}
\begin{tabular}[b]{c}%
\subbottom[A]{\rule{0.3\linewidth}{100pt}} \\
\subbottom[B]{\rule{0.3\linewidth}{100pt}}
\end{tabular}
&
\subbottom[C]{\rule{0.6\linewidth}{230pt}}
\end{tabular}
\caption{D}%
\end{figure}
\end{document}

I'd however, suggest minipages of fixed height inside the tabular (again, with bottom alignment):
\documentclass{memoir}
\newsubfloat{figure}
\begin{document}
\begin{figure}[H]
\centering%
\begin{tabular}{@{}lr@{}}
\begin{minipage}[c][230pt][b]{0.3\linewidth}%
\subbottom[A]{\rule{\linewidth}{100pt}}\\[9.5pt]
\subbottom[B]{\rule{\linewidth}{100pt}}
\end{minipage}
&
\begin{minipage}[c][230pt][b]{0.6\linewidth}%
\subbottom[C]{\rule{\linewidth}{230pt}}
\end{minipage}
\end{tabular}
\caption{D}%
\end{figure}
\end{document}
