If anyone is curious here is how I achieved this, however this is specifically tailored to Flash Builder/Flex Builder (as that's what our team uses) and unfortunately I could never get the ${eclipse.home} property to work in Ant so I had to use ${eclipse.pdebuild.scripts} to get at the installation directory:
<property name="install_loc" value=""/>
<!-- find the eclipse install location -->
<script language="javascript">
<![CDATA[
// Because ${eclipse.home} is not available, determine the install
// location using the pdebuild.scripts location
self.log("Looking for Eclipse installation...");
var base = project.getProperty("eclipse.pdebuild.scripts");
var path_pieces = base.split("/");
var path = "";
outterLoop: for(var i = path_pieces.length; i >= 0; --i)
{
if(path_pieces[i] == "Adobe Flash Builder 4" || path_pieces[i] == "Adobe Flex Builder 3")
{
// After determining which array item refers to the Adobe Flash Builder or Flex Builder
// installation, start at the beginning of the array and count up to that point, adding
// paths as you go.
var k = 0;
while( k <= i )
{
path += path_pieces[k] + "/";
++k;
}
break outterLoop;
}
}
// TODO: MAKE SURE THE PATH IS NOT EMPTY
self.log("Install path found at: " + path);
project.setProperty("install_loc", path);
]]>
</script>
<loadfile
property="workspace_prefs"
srcFile="${install_loc}configuration/.settings/org.eclipse.ui.ide.prefs">
</loadfile>
<property name="workspace_loc" value=""/>
<scriptdef name="find-workspace" language="javascript">
<attribute name="workspace_data"/>
<![CDATA[
// Find and return the workspace location
self.log("Looking for Eclipse workspace...");
var defs = attributes.get("workspace_data").split("=");
var loc = defs[defs.length - 1];
self.log("Workspace found: " + loc);
project.setProperty("workspace_loc", loc);
]]>
</scriptdef>
<find-workspace workspace_data="${workspace_prefs}" />
</target>