The Taylor brothers (who were experts in doing this) used a Bow-spring Mapping Pen I believe. ( http://www.classiclightweights.co.uk/de ... aylor.html
- and http://www.classicrendezvous.com/Britis ... Taylor.htm
Even Prince Charles had a go with one when he visited their premises once. I've seen a photo on a website somewhere.
The paint - which can be Humbrol as mentioned above, needs to be thinned slightly (like single cream) so that it runs smoothly. Adjust the jaws of the mapping pen to the thickness you want, load it with paint using a brush, wipe the excess from the outside - and try it out. If you have a rag moistened with thinners to hand you can wipe it off and try again. Practice makes perfect!
Oops, as usual, I didn't fully look at the OP and now I see it's about lug lining! A mapping pen can be used for this on plainish lugs but for more complicated ones (eg Nervex Pro etc.) a fine sable hair brush is better. However, as there is basically a long line to make, one with sufficient bristles to hold a reasonable amount of paint is needed. A 'Rigger' brush is therefore useful as this has a fine point but has long bristles that hold a larger quantity of paint to allow the line to be made without breaking. The original purpose was for ship painters (the artists that is!) to be able to paint the rigging of tall masted sailing ships. Obtainable from all good artists materials shops. Make sure you clean it thoroughly after use with white spirit and then soap and water and it will last you for years.
Today is the yesterday of tomorrow.