Is there a fuel injector cleaner additive available? I have not seen any additives (such as those used in automobiles) to clean and maintain fuel injectors in aviation engines.   Is there a reason for that such as the use of lead in 100LL or something else?   The fuel injectors will eventually constrict or clog as the avgas leaves residual deposits.

Is there some technical reason or is it just that no one has gone through the certification process or at least (like AVBlend or CamGuard) received the "will do no harm" approval from the FAA?

Removing, cleaning, reinstalling fuel injectors is not only expensive but you also risk damaging them and if you have the GAMI injectors you can have quite a bill coming at you.
Fuel injectors in GA piston engines are actually fixed nozzles, so short of fuel contamination, are not so easy to clog,since the only thing that can  be deposited with clean fuel is the blue dye in the avgas.

In 8 years maintaining a fleet of 6 to 8 injected Continental engines, running 200-700 hours / year, I did not see a fuel-deposit blocked nozzle. Deposits did change the flow rate enough for us to routinely clean them ultrasonically (our pilots were cruising lean of peak, and would start complaining), but removing, cleaning and reinstalling the injector nozzles was a routine, straightforward and scheduled event in my experience.   

With position tuned nozzles, as current Continental ones are, you need to be careful to get them back to their correct position, just like GAMI ones, but it's not rocket engineering. The torques for installation are in the manuals, and they are not the most challenging item to work on (one b-nut and one pipe thread per nozzle) on the average GA engine. There's no need for special additives.