Thanks for the info; would you happen to know of any way to force it to behave the same way as if the user logged in? Right now I have to guess at the files to explicitly source in to "fool it"....
I notice that su - user (note with dash) brings in more of the user's environment than does su - user -c 'command'. For example, if root does an su - user, and types "umask" to the prompt, one umask...