I wouldn't use any type of drop or pinpoint applicator for Neolube. It is watery thin, and no matter how small the applicator, I think you'll find that it runs out like crazy and makes a mess.
Very small artist brushes are the way to go. You can get some on the brush, and reach way in
to anywhere you need to go without making a mess. When I did my NP W-5, you may have noticed all that piping all over the smokebox. I Neolubed that smokebox AFTER all the piping was on and the engine was painted, and I was able to get in, around, and behind it without any problems.
See this photo and look at the silvery gray smokebox with all the piping on it, especially on the sides. And note the joint/rivet line between the smokebox and the rest of the boiler where it goes from paint to Neolube. All that Neolube is just carefully brush painted after the whole body was painted and detailed. I think a pinpoint oiler would have let it run all over the place.
You can clean the brush with alcohol afterward.