Another idea which someone may have time and resources to try out before I can:
One of the barriers to building an injector is the problem of having a plunger which fits perfectly inside the barrel. This is difficult to achieve without a lathe. However, a single-piston combustion engine such as a motorbike engine has both a barrel and a plunger perfectly milled to incredible precision. Maybe we can take the spark plug out and put a nozzle instead? A heating element could be wrapped around the engine block, between the cooling fins, and the cylinder filled with plastic, melted and plunged with the piston.
Maybe I’ll get to try this, maybe not.