In any case this seems more like competitive advantage through obscurity than security through obscurity, for the same reason that manufacturers don't include blueprints and engine schematics.
Engine tuners already do this.
But if somebody could rewrite the firmware so that the car stereo controls all of a sudden are the only way to actuate the brakes, that could create very dangerous situations. Which I why I hope that there will remain strong regulations on what firmware people can load onto their cars, at least in as far as they use that car on public roads.
Aside from some minor pulsing of the ABS module, it's a purely mechanical/hydraulic system. You push the pedal in, the master cylinder creates pressure in the line, and the brake caliper pistons get pushed out onto your brake rotors.
On the other hand, as seen in the Pruis nonsense last year, an increasing number of cars do use a electronic throttle which can lead to some interesting failure conditions.
The 'firmware' your discussing in your hypothetical situation is hacking the CANbus system - not the ECU firmware. One could, in todays systems, remap any CANbus device (even as odd as a headlight switch) to manipulate another CANbus device (such as the electronic throttle). Doing such nepharious things are possible today, but would take days worth of hacking and proprietary diagnostic hardware.
ECU controls, on the other hand, have been reverse engineered in nearly all cars being actively tracked/raced today. Bosch (BMW/Porsche), GM PCMs, Honda/Acura, Mistubishi, etc can be modified to control timing/fuel...add additional I/O and features (such as traction control, launch control, antilag, etc ...even when not equipped by the OEM).
Of my vehicles still running an OEM ECU, all have been running 'hacked' firmware without incident for nearly a decade. Even my ABS controller has been upgraded for better threshold braking performance.
If you're intersted in such things, please check out EFIuniversity [0], Ostrich, and persuse Motec's manuals for an idea of what's possible.
[0] www.efi101.com
[1] http://www.moates.net/documentation.php?documentation_id=24
[2] www.motec.com%2Ffiledownload.php%2FM4_M48_M8_Manual_A5.pdf
You have to be in fake-manual mode, set to 1st gear, with the brakes depressed, and the box enabled. Then you pull back on the cruise control stalk to engage the brakes. There's really no way to accidentally engage the mode, but yeah, that sounds exactly like the sort of thing you're worried about. Using the cruise control stalk to trick the car into thinking it hit a patch of ice, so it selectively engages the brakes.
I should note that a) I don't do burnouts and would never use this option, and b) I believe you can enable/disable it via USB and the programming utility.
Happy driving ;)
Of course, should something happen, there's a number to call to get towed away quickly.