Nick and anyone else looking to retrofit: Tesla has a TSB on how to add the RFS if your car wasn't ordered with it. Tesla won't install it for you, but it is an easy install, there is a thread here somewhere with the TSB links, photos, and a walkthrough that makes it easy.
You can also contact a Tesla-certified third party shop in your area and have them do the install if you don't want to DIY.
The hardest part is sourcing the parts; this thread is teh seats and hardware, so make sure it has the bolts and strikers, and if possible the bumper insert and interior door switch, and that's everything. Anything missing can be sourced on ebay or here later.
There are literally only 20 early Tesla Model S cars that can't have a RFS installed at all, the rest can be retrofitted.