Dammit I forgot that Samba can "do" AD-DC these days - thinks, get one of those low-powered mITX boards and slap on a copy of Ubuntu Server onto a bootable CF and jobs a good 'un. Think I just found my project for those long winter months...
That was kind of my point - the old way you could unpick an SP download (extract the CAB?) and merge it into the main code.