I've come to the conclusion that it is a doubtful approach to take the mtime and ownership from the file and use it on the upper directories as well. So, I've come up with a totally different solution (cp. makedirs.diff) that abandons the use of os.umask() completely and uses a single call to os.makedirs() to create the missing directories.
It seems very attractive to me to do it this way, what do you think?
File Added: makedirs.diff