FX is an input/output stream, so unless you can find the right cable AND software that will allow you to use both the mic and the headphone connections in that jack, you are probably out of luck. Alternatively you could use either an external USB sound card (with the Camera Connection Kit) or experiment with something like the Alesis iO Dock II.
http://www.alesis.com/products/view/io-dock-ii
The second challenge is that in studio terms, external sound processing (i.e. FX) is often mono. So I am not sure what FX apps exist that run on an iPad and allow you to process stereo inputs.
The iO Dock is at least something that gives you full high-quality audio output from your iPad while being able to charge it at the same time and it would make hooking things up easier than through the TRRS connector of your iPad.