I think 'release' is the last target that can run. You could just add a copy command in the release target to copy the apks to your folder. I've moved to SDK 4.0 which has a different build.xml structure, but when I was using the old one, I had a section in my build.xml's release target
<!-- Zip aligns the APK -->
<zipalign-helper in.package="${out.unaligned.file}" out.package="${newout.release.file}" />
<echo>Release Package: ${newout.release.file}</echo>
<echo message="Copying the googlemapdebug.xml back to googlemap.xml" />
<echo message="ALWAYS LEFT IN DEBUG FROR ECLIPSE" />
<copy file="${layout.dir}/googlemapdebug.xml" tofile="${layout.dir}/googlemap.xml" overwrite="true" />
which copied a single file back after a release build (I'd defined layout_dir in my build.properties and before the compile I'd previously copied a googlemaprelease.xml which had my GoogleMap release API key in it).
You could adapt this and have your apk copied to a folder specified in your build properties