Yes, set it in a \vbox of a fixed height (so that the same height is used by all chapters; this means you need to set the height to the height of the largest epigraph you will use), eg
\chapter{Stuff}
\epigraph{blah blah}
where \epigraph is defined something like:
\newcommand{\epigraph}[1]{\vbox to2cm{\noindent\small\itshape#1\par\vfill}}
However, as chapter titles can also be long, and run to more than one line, you may want to take them into account as well, and rewrite \@chapter entirely so that it typesets the chapter title and the epigraph as a single block of known height, so that the text always starts in the same place. That way you could write
\chapter[short version]{Something very long}{An epigraph text here}
If you need to provide a proper citation for epigraphs, you'd need a fourth argument. If it needs to be optional (some epigraphs have them and others don't) then you need to use the xargs package...and it just gets more and more complex the more you need to automate :-)
///Peter