You can redefine the caption building command so that the captions have zero height.
\documentclass{book}
\usepackage[a6paper,margin=10pt]{geometry}
\usepackage{dpfloat}
\usepackage[demo]{graphicx}
\makeatletter
\long\def\@makecaption#1#2{%
\vskip\abovecaptionskip
\sbox\@tempboxa{#1: #2}%
\ifdim \wd\@tempboxa >\hsize
\vbox to 0pt{#1: #2\par\vss}
\else
\vbox to 0pt{\hb@xt@\hsize{\hfil\box\@tempboxa\hfil}\vss}%
\fi
\vskip\belowcaptionskip}
\makeatother
\begin{document}
Some Text
\begin{figure}[p]
\begin{leftfullpage}
\centering
\hrule
\includegraphics[width=5cm,height=3cm]{inserthere}
\hrule
\caption{This is the short caption.}
\end{leftfullpage}
\end{figure}
\begin{figure}[p]
\begin{fullpage}
\centering
\hrule
\includegraphics[width=5cm,height=3cm]{inserthere}
\hrule
\caption{This second caption does not only look larger, it
actually is, by one line more.}
\end{fullpage}
\end{figure}
\end{document}

Enclose the redefinition in a local group including the figures to affect only these captions.
Of course you may want to regard the caption as having a different height. The following sets up an environment fixedheightcaptions within which all captions have a definable height, default 0pt.
\documentclass{book}
\usepackage[a6paper,margin=10pt]{geometry}
\usepackage{dpfloat}
\usepackage[demo]{graphicx}
\makeatletter
\newlength{\@fcaptionheight}
\long\def\@makefixedcaption#1#2{%
\vskip\abovecaptionskip
\sbox\@tempboxa{#1: #2}%
\ifdim \wd\@tempboxa >\hsize
\vbox to \@fcaptionheight{#1: #2\par\vss}
\else
\vbox to \@fcaptionheight{\hb@xt@\hsize{\hfil\box\@tempboxa\hfil}\vss}%
\fi
\vskip\belowcaptionskip}
\newenvironment{fixedheightcaptions}[1][0pt]{\setlength{\@fcaptionheight}{#1}
\let\@makecaption\@makefixedcaption}{}
\makeatother
\begin{document}
Some text.
\begin{figure}[h]
\centering
\hrule
\caption{Test caption, normal}
\end{figure}
Some text.
\begin{fixedheightcaptions}[5cm]
\begin{figure}[h]
\centering \hrule
\caption{Test caption of height 5cm}
\end{figure}
\end{fixedheightcaptions}
Some text.
\begin{fixedheightcaptions}
\begin{figure}[p]
\begin{leftfullpage}
\centering \hrule
\includegraphics[width=5cm,height=3cm]{inserthere}
\hrule
\caption{This is the short caption.}
\end{leftfullpage}
\end{figure}
\begin{figure}[p]
\begin{fullpage}
\centering \hrule
\includegraphics[width=5cm,height=3cm]{inserthere}
\hrule
\caption{This second caption does not only look larger, it
actually is, by one line more.}
\end{fullpage}
\end{figure}
\end{fixedheightcaptions}
\end{document}
Here is the first page of output:

\\ \mbox{ }. (if you have a list of figures, you'd have to provide alternate text for that list with the optional argument to\caption, omitting the extra stuff.) – barbara beeton Oct 2 '12 at 15:50Argument of \@caption has an extra }is produced. With optional argument, the document compiles find but the\\seems to be ignored in the caption. Only with thecaptionpackage, an additional line is produced but apparently only if there is something visible after the newline.\mbox{ }did not work. – sebschub Oct 2 '12 at 16:17\mbox{ }you used a rather substantial\phantom{...}? – barbara beeton Oct 2 '12 at 17:26\vphantom{...}with a depth equal to the baselineskip of the font used in the caption. put it right at the beginning, so it doesn't affect the centering of the short caption text, and add the alternate text for the list of figures. or ... what about simply adding a\vspace{1\baselineskip}after, and outside, the caption. – barbara beeton Oct 2 '12 at 18:35