The engine is designed to drop revs as quickly as possible... applying brake on the alternator, so i don't believe it acts as a flywheel in this case.
I would consider a simple switch if:
- i would be using a belt driven water pump.
- i could easily charge my battery at night (car is in a busy street without power outlet)
It would be to easy to forget the switch (like the many times i forgot to switch off the choke). With the possibility to overheat the engine in my case.
and... where would the gain come from if manually switching on and off? Are you really just switching the alternator on every time you use the engine for breaking? Couse that is the only time you can use power that is lost otherwise. And your allready using it in a standard setup. The real gain in that setup is by using the alternator as 'emergency unit' and using cheap electricity to charge batteries.
Point is to use the power from engine braking to spin a flywheel, and use it to produce much more electricity (because it will stay spinning without applying a load on the engine) than possible with a conventional system.
flywheel and clutch are very common, and the setup is easy to fabricate. (having a lathe to fabricate pulleys)
Using a centrifugal clutch (like in a chainsaw for example) would even make it possible for the engine to spin freely (without power loss caused by the added flywheel) at idle and making the clutch engagement smoother. And acts also as a freewheel when engine revs drop.