Not sure about a lightbox, but you can create a login page that allows you to manage it and the content on it (steps below).

Step One (Create Page)

1. Login a Admin or Host
2. Click the Add Page button (top left of web page)
3. Enter a Name/Title you want
4. Allow "All Users" to have view rights
5. Remove the checkbox from the "Display in Menu" checkbox
6. Click Update

Step Two (Configure Page)

1. Select "Login" from the module dropdown list (top center of page)
2. Give it a Title
3. Click the Add link/button
4. Add other modules you want (such as welcome text etc)

Step Three (module permissions each module you have added)

1. Select Module Setting from the module Action Menu
2. If select either "Unauthenticated Users" (login module) or "Registered Users"
   Depending if it should be seen before or after login (i.e. directions or welcome)

Step Four (Set Login Page in Site Settings)

1. Go to Admin > Site Settings
2. Expand "Advanced Settings" > "Page Management"
3. Use the "Login Page" dropdown list to select your new login page
4. Click "Update" to save

NOTE: Be sure the login module is loaded onto the page and has the correct permissions before making the site settings change.  Once you save the settings you will only be able to login using that page and if you have not setup the page/login module correctly, you will have no easy way to login...

Best Regards,

Robert J Collins | Co-Founder & President

Netlogic Corporation