How about mig welding nozzles?
Most common sizes just use a M6 thread. Suitable hose, adapter (just a hosetail with female M6 for screwing in the tip), and some method of securing the hose (similar type of arm as used on DTI gauges?).
That way you can try different sizes (0.6, 0.8, 1.0mm are all common sizes), and you can try modifying them to suit without any major outlay.

Other option could be syringe nozzles (I know they're available in a very wide range of sizes for applying solder paste), but making an adapter would be a bit more challenging, as they normally use a 1/4turn type moulded fitting.