[I've cut down the To: and CC: headers to olny include python-dev and distutils]
Well, site.py could be modified to set a symbol in the sys module which could then be queried by distutils, e.g. sys.extinstallprefix.
Alternatively, distutils could be made to default to Lib\site-packages and then revert to Lib\ in case this directory is not available.
BTW, I don't think that using Windows registry keys for determining the installation path is a good idea -- this information should be kept in the site.py or sitecustomize.py module for easy editing.
The problem is that the 'installation path' information must be loaded at run time by the windows installer, and it may not always sucessful to embed python at run time and let python code retrieve it. Remember the problems we had with Python2.0 on win95/98, when win32all was not installed? The installer was not able to compile the installed files to pyc/pyo because of this path bug.
Anyway, how does bdist-rpm does it? Should be the same problem there...