Get parameters are basically standard for any Web API's so you will probably find that you will end up using them a lot so rather than hack fixes for each and every API create a middle man.
Create a new folder at the same level as your index.php (usually your site root). call this folder something like middleman
Add the middleman
folder to the list of items in your .htaccess
file that should not be routed through the index.php
Now for every API you use you can create a new .php file in the middleman
folder which transforms all requests to one that your site can understand.
You then point your external API's to http://yoursite.com/middleman/api_name.php
The reason for creating the middleman is that Code Igniter obliterates the get array so it gets removed before you can process the requests and format them into something meaningful if you try to do it inside Code Igniter so we must perform the transformation outside of Code Igniter's scope.
an example file might looks like this:
mysite.pingback.php
<?php
$from = $_GET['ip'];
$time = $_GET['time'];
$post = $_GET['id'];
header('location: ../mysite/pingback/'.$from.'/'.$post.'/'.$time);
?>
So very simple files. but it also means that when a change is made to the external API it does not affect how your Code Igniter app works as you simple have to change your middleman.