In case anyone else finds this thread and wonders if a resolution was ever found (like I did), here is some info from Apache (Core Features)
"In addition, if you use a remote URL in an ErrorDocument 401, the client will not know to prompt the user for a password since it will not receive the 401 status code. Therefore, if you use an "ErrorDocument 401" directive then it must refer to a local document."
Therefore, if you want to redirect a 401 error, you must use something like this in your .htaccess file:
ErrorDocument 401 /err401.html
You cannot use these:
ErrorDocument 401 http://www.someweb.com/err401.html
ErrorDocument 401 http://1.1.1.1/err401.html
Hope this helps.