"I think paraffin could be bad for the machine, having such low viscosity means it's going to get into everything."

Paraffin is no worse than standard water-based coolants in this regard.

What I have done before today is to put some paraffin in a bottle (500ml Coca Cola bottle or similar), with a small hole punched in the top, and squirted it when and where I wanted it, that way you don't have a huge amount of mess to clean up (Just make sure you identify what's in the bottle so it doesn't get mixed up...)

Not sure I'd be using brass for Injection moulding though, I'm not convinced it would work.

What temperature you anticipate running your moulding machine at?