There should be. There's a few options you have there...
1) If you want to popup the Facebook login window at that time, you could with the following:
'javascript:alert(\''.JText::_('JGS_COMMON_ALERT_NO_DETAILVIEW_FOR_GUESTS', true).'\');jfbc.login.login_custom();';
It's possible I messed that up with all the punctuation.. it should look like:
javascript:alert('hi');jfbc.login.login_custom();This will alert the user "You must be logged in" and then actually pop up the Facebook login window. It won't allow them to login with Joomla, but they could close the window and log in that way if they wanted.
There's no way to put the button in the text box itself, but if you update the language string above to say "Access denied, a FB window will appear after this to allow entry in one-click... ", that may do it for you.
2) You can simply make the Login Button show on the page somewhere and update the string to say "You must login in, either go 'here' or click the Login With Facebook button for quicker access...
Hope this helps give you some ideas, and good luck!