Transient voltage protection on an automotive circuit
Wondering if/how you solved this.  I agree w/ mkeith above.  You don't need to kill the 12V to the coils, you need to disallow it's interruption by bypassing the system transistors.  You could do this by feeding 12V thru one large(er) MOSFET to all coil 12V inputs - thru diodes (so the coils don't crosstalk.)  It would likely create a puff of black smoke and/or a tailpipe backfire.  You're dumping raw fuel/air out into the exhaust and then firing fully (air/fuel)loaded cylinders after that.  The flame will leak out of the exhaust valve and ignite what's' in the exhaust pipe.  Sounds like noisy fun if you don't have mufflers to worry about.  Backfires (exhaust explosions - not intake) will usually split the muffler depending on the construction.  You may also want to blank out your injectors during this time.  Injector blanking means lean cylinders but no spark to overheat... but power recovery might be a problem as the cylinders need to be reloaded with fuel once normal operation is restored (clutch released.)  Your control switch could be a basic contact switch on the clutch.  This could also be completely analog /w no micro(Arduino)...and thus no software...and thus faster.  If you add the injector blanking function, you can always use it, or not.  Injector blanking would involve denying drive signal to each injector.  If they are individually fired you need a series switches(MOSFETs), one for each injector.  If they all fire at once, then just interrupt power to all of 'em due to their only running on one channel, will suffice.  If you're on carbs, your out of luck.  ...Good luck.