The Make Delphi action determines that path by finding the highest version key in the registry under
HKEY_LOCAL_MACHINE\SOFTWARE\CodeGear\BDS
(for D2009, it's 6.0) and reading the RootDir value from that key. You could temporarily update this registry value to point to D2007 before building D2007 projects, then change it back to the D2009 path afterwards.
http://www.kinook.com/VisBuildPro/Ma...teregistry.htm