Most often people will use an old brush, incredible nib or Masquepen but depending on the flow, coverage and detail you want. There are a couple of other things you can try.
Lets say you want a nice continual flow of fluid to leave detail areas white. Consider an embossing tool (used for paper, foil, clay etc) or a calligraphy pen, both feel good in your hand and hold a nice amount of fluid. The flow of masking or drawing gum can also depend on the brand you use. Personally I like Pebeo drawing gum because it's thinner and applies easily.
Examples from students
When signing paintings with a dark backgrounds Karen Richards like to use a Calligraphy pen to her apply masking. Once the composition is created she applies a light wash, lets it dry then signs her name. She will then continue with the painting then when done remove the masking to reveal her signature.
Pebeo Drawing gum
To get basically the same results Linda likes to use an embossing tool. Used mainly for paper, foil, clay, leather etc the embossing tool is about 5 1/2 inches long with a wooden handle, stainless steel tips with a small round balls on either end. It holds a nice amount of fluid and flows well.
Embossing tool with wooden handle, stainless steel tip with a small ball on the end
Applying Pebeo masking fluid