Congrats on the event. Just finishing is an accomplishment, let alone not being DFL.
Off hand, I don't think Subaru was ever big on front LSDs. You're probably looking at an aftermarket diff, as I'm not so sure that an STi diff is a good idea in a FWD car simply because it was designed specifically to work with the other two diffs in the STi. Sure, it would probably be an improvement over an open diff, but you'll probably be spending a pretty penny to get one, in which case you can probably get a Cusco diff tailored for what you're doing for not much more. And if the STi LSD can't hold up under FWD rally conditions, well now you're not just spinning the inside tire, you're DNFing.
You might want to get in touch with the RalliSpec guys, or Jamie (SubieGal on NASIOC), they can probably point you in the right direction... just don't let KevinM know you're talking to SubieGal.