Whats the best way to share user data across multiple Wordpress websites?

Question

First of all, Multisite isn't what I'm looking for. I have 5 websites using Wordpress all on separate domains that I would like to enable user accounts for. I need user data to be shared across the entire network and when a user creates an account, they would have instant access to all other websites as well. What I'm trying to do isn't recommended by Wordpress when using Multisite:

If you plan on creating sites that are strongly interconnected, that share data, or share users, then a multisite network might not be the best solution. -http://codex.wordpress.org/Before_You_Create_A_Network (1st paragraph, 3rd sentence)

I've tried using the "shared user table trick" as mentioned here http://wordpress.org/support/topic/multiple-sites-same-users-how and here http://xentek.net/articles/528/implementing-the-wordpress-shared-users-table-trick/ and a number of other places...the problem is that this information is 2-7 years old and I'm using Wordpress 3.5.1 and this "trick" doesn't seem to work.

I've attempted to insert this code into wp-config around the $table_prefix line.

define('CUSTOM_USER_TABLE', 'wp_users');
define('CUSTOM_USER_META_TABLE', 'wp_usermeta');

Long story short, I don't think this trick works any more as I am unable to log in to the second site on the admin side at all.

Any suggestions on what the best way to link this data would be?