You are extremely fortunate in that there was an open gate you could go through. Glad you are OK. I experienced something similar at Watkins Glen in an E92 M3 two years ago after too many hot laps with the stock system + fluid + HT10 pads + SS lines. I was fortunate in that I ended up in a gravel trap.
I tracked the Z4M only twice, and was more careful with it, but I still saw similar issues with overheating brakes if you push it in a prolonged manner. Of course, it doesn't help that you've got a supercharger on yours.
I don't know if your brake system was completely stock, but I assume you had track pads and fluid in there?
I made several posts reporting rotor temperatures and saying be careful with the stock brakes, but got several responses saying the stock system is fine (not speaking to the OP here).
(Also, please don't start the "just don't overuse the brakes" line because it is a bogus argument for the most part; the faster you lap a track, the more heat you put into your brakes because you approach the braking zones at higher speeds and E_k=0.5*m*v^2.)
No, the stock system is absolutely not fine if you push the car in a prolonged manner--even if you have track pads and fluid. Be careful out there. If you are not sure about what's happening to your brakes, at least measure temperatures with an IR gun at the paddock when you come in. If you are seeing anything above 550C at the paddock, that is reason for concern and you should back off...
Something similar happened at Spa to a pro driver in a E92 M3 here:
http://www.m3post.com/forums/showthr...+brake+failure