The application isn't as restrictive as you think in race circles,all of the old air cooled kawasakis seem to have twin plug heads and quite a few of the Suzukis do too.So you would be able to get a system on one of those bikes.
The problem here is where and what are you going to run it on?
As I see it there are a number of problems with the current nitrous systems that need to be addressed.
1. Uniformed approach to the problem,theres no single method thats accepted to be the best at the moment.The wet systems all have issues that get bandaids stuck on them to make them work,small lines big lines jets at solis jets at foggers,fogger1,2 softplume,crossfire etc etc.High fuel pressure low fuel pressure,modulated fuel soli not modulated fuel soli,one fuel soli per cylinder!!Progressive build up in time for whole run ,time per gear,rpm ,staged plus any previous combo etc etc.Go in any pit at a race meeting and you will see a different approach of how to do it.
With a Dry system a lot of the drama goes away leaving you with the ECU chioce and soliniod choice.Although it does open up a whole new World of control,with a number of options for how the nitrous comes in.Also the choice of multi staged or Revo.
2.The Turbo World on bikes is a lot more straight forward Turbo at the front close to the exhaust ports as poss a tube up to a plenum EFI for the fuelling, select your turbo by how much power you want and dial your progressive by each gear with boost ramps then when you're really clued up by time with ramps.
Somewhere you need to make the transition from nitrous wet with all the probs and arguements as to whats best, to dry systems where all you need to argue about is the application of the progressive gas.Currently in the dry field you have it the ultimate answer the 'Revo' I have an interim solution of multistaged,although you too could configure that with maximisers anyway.
Get this job done and get at least one vechile to break a record then you could run a direct feed system as you suggest above and you WILL be listened to.
But direct cyclinder nitrousing up a Smart car is something I may consider but wouldn't neccessarily publicise.If I were looking for a development path to nitrous etopia I would consider the following.
1.Dry nitrous system with piggy back ecu
2.Dry nitrous system using Revo
3.Direct to cylinder nitrous system.
All fitted to a race vechile that runs the numbers.
Although I'd keep the wet stuff in my line up I would stop all development work on wet nitrous systems as of now! its a problem that can be solved but the race customer base is full of 30years lucky blue underpants solutions that you will never succeed in converting the masses,like I say latest bandaid I heard about in the bike pits was a fuel soliniod per cylinder what ever next a fuel injector per cylinder maybe? they'll get there in the end I guess.