Feature Request: Additional Merge Fields

This might be outside the scope of this plugin, but it'd be really cool to let the network admin choose additional Merge Fields for the MailChimp plugin. For example, it'd be really useful for me to merge in the user's site URL or username, so I can use them in outgoing emails.

Now, I realize that a GUI for this would be quite a bit of work, but how about just adding a filter to the plugin. Something like this in the mailchimp_add_user function:

$merge_vars = array( 'OPTINIP' => $_SERVER['REMOTE_ADDR'], 'FNAME' => $user->user_firstname, 'LNAME' => $user->user_lastname );
$merge_vars = apply_filters('mailchimp_merge_vars', $merge_vars, $user);

I'm going to add that myself… but I'd hate to have to manage a copy of the plugin that I have to merge each time you release an update. Thoughts?