We use generated custom fields and want to see them it on the view issues page too , so we need the bug_id as parameter in the user function call:

in function string_custom_field_value:
original:
return call_user_func( $g_custom_field_type_definition[$p_def['type']]['#function_string_value'], $t_custom_field_value );
fixed line:
return call_user_func( $g_custom_field_type_definition[$p_def['type']]['#function_string_value'], $t_custom_field_value , $p_bug_id);

if that's possible please add this to the core, because we need always patch it after the upgrades, and i think it could be useful for the others too.