ACF5 Pro ( Advanced Custom Fields ) licence activation. Could be adapted for any other plugin that requires a licence key, but doesn't yet support defining it in wp-config. Fires on theme activation.

licence activation.php

// Place this in wp-config
define('ACF_5_KEY','yourkeyhere');

// Set ACF 5 license key on theme activation. Stick in your functions.php or equivalent.
function auto_set_license_keys() {
  
  if ( !get_option('acf_pro_license') && defined('ACF_5_KEY') ) {
    
    $save = array(
		'key'	=> ACF_5_KEY,
		'url'	=> home_url()
	);
	
	$save = maybe_serialize($save);
	$save = base64_encode($save);
      
    update_option('acf_pro_license', $save);
  }
}
add_action('after_switch_theme', 'auto_set_license_keys');

en el wp-config no sirve, enseguida da error el cms