Getting back on topic.
Thinking about things, going back to first principles - you can't get something for nothing.
I think the idea that has the most merit is directly injecting fuel into the manifold that feeds the turbo -- making a little jet engine effectively.
Basically, you'd need to add a fifth injector, which has very different firing profile from your other injectors.
To make this effective, and to keep your turbo alive for a long time, you'd have to make some changes -- effectively increase the heat capacity/tolerance of your system. I think the easiest way to do this, would be to put some special high temperature ceramic coating on the *inside* of your manifold and on the hot side of your turbo, and on the downpipe, all the way up to the catalytic converter (wait, you still have one of those, right? )
Why? -- keep the darn turbo spooled all the time - make it a fricking JET engine.
You'll need the fuel injector from a diesel, that's the only thing that can take the temperature/pressure in the manifold, and a high pressure injector pump - again, from a diesel. You should be able to run high octane gas, straight in to the manifold.
If you've got your manifold/turbo properly coated (the most expensive thing), then you'll be able to take the extra high temperature which will result from running the turbo as a jet engine.
Once the turbo is spooled - you don't need to inject any more fuel.
You probably don't want to inject any extra fuel when coasting.
I was trying to think about this, while in the shower, in the following terms:
1) You can't get something for nothing.
2) Heat drives the turbo. Increase the heat to spool the turbo, keep the heat constant.
3) How do you increase heat? Dump fuel into the exhaust manifold !
4) How do you do that safely??? Just a little, at the right time, take proper precautions
5) Weld, Coat, Take pictures, Enjoy!
In other words, I think before I was wrong, and that the bang-bang systems are the right idea. Really, the only way to do it well. Don't bother with the wasted spark and extra injectors, just shove some more fuel directly into a prepared exhaust manifold and enjoy!