Answering my own question to some extent, in case anybody ever comes after from google:
A bit of source diving shows me that there is an undocumented property for <jdbcconfiguration> in ant: preferBasicCompositeIds .
However there is a comment against it, "not exposed here", and perhaps unsurprisingly in the light of that, it doesn't seem to make any difference when you set it, which is unfortunate.
So it appears this functionality is exposed to the GUI reveng tool, but not to the ant tasks. So I might as well wave the white flag on this and continue manually generating.
Source I looked at:
http://anonsvn.jboss.org/repos/hibernat ... nTask.java