I remember BearFoot Arts ( the maker of the electric Mandrel Spinner { EMS } ) had small vibrating brass tubes with a variety of sizes of tiny openings that would dispense enamel powder in extremely fine lines that you could roll your hot bead over to pick up.
Just a sec....
Here is a youtube video from 2008:
https://www.youtube.com/watch?v=Ou33iBrQjf0
I would not be surprised if he has not sold this product off to Diamond Tech Crafts like the EMS mentioned above.
The book he mentions in the video by Cindy Jenkins should have more details on other techniques too.
If you wanted even more control I would try encasing the enamel in clear glass and then pulling fine stringer from that.
Let us know what you learn please.
ETA: His method of making a crease / valley in the hot bead and then applying enamel to that channel could get an even finer line if you then heated the bead enough for the channel to close back up. That should leave a very fine line where the two sides of the channel come back together.
Here is another video of his on these Vibe things;
https://www.youtube.com/watch?v=uSI15LtZFt0
And yet another update;
https://www.youtube.com/watch?v=lDdt11xtOeo
Yet another ETA:
Here is his web site;
http://www.bearfootart.com/category_...Frit-Vibes.htm
Seems he has updated the older version of this Vibe tool and has ones that will draw lines with the smaller frits as well as enamels.