Keep in mind that at high loads/WOT, the injector may be open nearly constantly - during all four strokes. (Injector Duty Cycle >90%)
In fact, many systems switch from sequential port to bank and then continuous fire (all injectors firing at the same time) when high fuel demands are called for.
The difficultly is balancing WOT needs with idle with only one injector. Big pintles and nozzle orifices mean bigger flow, but also add mass and latency to the operation. In addition, atomization efficiency is drastically affected by critical orifice size, and not as much by pressure drop. A firehose is great for fighting fires, but not as useful when adding a drop of water to your Scotch.
I'm not aware of a "variable" injector that would be cost effective in all modes. However, new GDI technology obviates a lot of those needs, and also has the benefit of not injecting on to a cold valve and destroying nice small droplet size. It also has the beauty of not caring about valve/cam lobe position. You can inject a very lean charge, and then a small pocket of stochimetric mix immediately before (and maybe even during!) ignition.
So my quasi-answer is that I can accomplish so much more with Gasoline Direct Injection, that although your idea is salient, I think those IC wizzards have already moved well beyond such ideas. In fact, they are just scratching the surface. I was invited to a webinar called "Dr. Stochiometric is Dead". I couldn't afford it, but my point is I fully expect IC efficiency to improve dramatically in the next five years. Just gotta deal with heat and NOx... Lean is the new phat!
To clarify, I only mean that an on/off style injector that is optimized for idle may not have enough ultimate flow for WOT. Albiet WOT is rare (well, for some... I won't specify what category I serve time for) the problem is you just can't go wicked lean without terrible consequences.
A good compromise might be a small-pintle small-orifice injector for idle, coupled with another injector: my proverbial "firehose" for hole shots. This, of course, would be problematic in engineering and production expense.
This is why I'm such an advocate of GDI, which allows a huge flexibility in injection schedule, regardless of valve position or which stroke you're on.
The idea of a variable-pressure fuel rail is laudable, if it only worked. As I mentioned before, when you look at optimized atomization based on a "critical" orifice, there's some strange physics involved. Orifice size rules, and pressure drop becomes almost fixed. Once an orifice is "critical", increasing feed pressure does not have the linear effect one might hope. (Diesel injection aside; that's a wholly completely different animal...) In addition, I'm not ready to admit (as if I understood!) that injection during an entire intake stroke leads to more homogeneous mixture (after compression) than just a short spray.
Idle emissions are at an all-time low. In Los Angeles, tailpipe emissions at idle are lower NOx values than the intake air.
Look to GDI and some crazy wicked power transmission technology to change the rules. Run your IC in a sweet torque band, very lean, and allow a seamless power transmission with low losses to allocate the torque. Save stochiometric for stoplights and when I'm trying to impress a girl...
(No, I'm not buying a Nissan... In my opinion they are virtually undrivable and too frustrating to comprehend...)