Sprint format racing, such as the SCCA's Pirelli World Challenge, is a natural choice for EVs. The races are short enough (usually no more than 50 minutes) that the biggest Achilles heel of electric vehicles, range, normally doesn't come into play. But the ultimate test of endurance and reliability seems to be just out of reach of performance oriented EVs such as the Tesla Roadster and Model S. Even with the supercharger's impressive 120 kW in 30 minutes, a LMGTE spec Model S race car would still spend about a third of its time in pit lane. That's almost as bad as the Delta Wing.
Way back in the before times, long before Jeremy Clarkson panned the Roadster for committing the unforgivable sin of being an American car, Elon Musk was floating around the idea of having removable battery packs for the then still nascent Model S. The idea was that you'd drive up to a battery swapping station, park your car over a hole in the ground, and a robot would swap out the entire depleted battery pack from your Model S and install a fresh, fully charged one.
For various reasons Musk seems to have given up on the idea, but the essence of his idea - removable battery packs - might be suitable for endurance EV racing. Tearing up pit lane at Circuit de la Sarthe to install a giant subterranean battery swapping robot probably won't fly with the track officials, and since the Model S has a proclivity for catching on fire when it runs over objects, removing the battery packs from directly underneath the car and placing them in a box similar to a racing fuel cell might be the best way to go.
A battery company called EnerDel based in Greenfield, Indiana seems to have come up with just such a solution with their series of Vigor+ battery packs. EnerDel claims that these 200+ lb. battery packs have "(r)obust automotive-grade packaging and built-in control redundancies (to) ensure enhanced pack protection and user safety." I don't know if this translates to FIA crash approved battery packs, but it's a good start.
I could envision a hypothetical "Model S.r" having a 40 kw pack bolted into the rear trunk with a second one bolted into the "frunk." A typical pit stop would go something like this: The Model S.r pulls into pit lane where it is immediately surrounded by four wheeled engine hoists, two that are empty and two with fresh battery packs. The pit crew opens the frunk and rear hatch so gunmen can begin unbolting and disconnecting the battery packs. The empty hoists hook on to an eyelet at the center of each pack and lift the old pack out. The process then begins in reverse to install the fresh pack.
I couldn't begin to accurately estimate how long this would take, but if a NASCAR pit crew can remove and install 4 tires with 5 lug nuts each and fill a race car with gas in less than 20 seconds, then I can't imagine a well trained crew taking much longer, especially if the packs are built and installed for racing. This video of an Audi Racing practice pit stop shows that it takes about 23-24 seconds to completely fill up a P1 tank, so it might be possible for a Model S.r with a well trained crew to pit in an endurance race while still being competitive.