Sure, you can put a drop of neem oil on a dog collar. I don't know how effective it would be but it's an interesting experiment.
Is there any particular reason why you want to use only one drop and only on the collar?
You could rub the oil over the whole collar. (If it's a leather collar it will help keep it soft and pliable...)
Or why not simply rub some neem oil into the dog's coat? That would repel the bugs better and it's good for the dog's coat and skin as well. And it makes the hair nice and shiny...
That's what I do on my dog every now and then. I rub a bit of oil in my hands and then rub the hands all over her back etc. (I also always spray her while I'm spraying the garden anyway.)