Here's a thought: Diesel injectors. They have the pintle already, can withstand ridiculously high pressures and temperatures, and are easily fast enough. The only issue I can see is that diesel is a lot less viscous than plastic...
Diesel injectors? You want to spray the plastic all over the place?
I'd say we'd need micro-injectors with piezos to get a manageable sized multi-nozzle footprint.
