I think also if you set the UID to under 500 it will hide it from the finder but not the loginwindow, if you use the previous line above with the sudo default write command it should do from both.
Sorry, I didn't fully read the original post. I take quick breaks from work and browse forums to clear my mind of something I am working on, and so as of a result I sometimes hastily answer things.
To answer your question about the user id, UID, you can set it by using the dscl command in OS X. For example, lets say you have a user called hidden administrator, short name hadmin
Code:
dscl / -create /Users/hadmin UniqueID 401
I am not exactly sure if you can modify a UID on a system, and I am willing to bet OS X doesn't like you doing that. I know in Work Group Manager in an OD environment you can't change the shortname nor can you change the UID once it has been generated. So, you may have to recreate your hidden user account.
