paint in a pen?
 

can you get any pens that are fillable with paint (like a black marker)?

was thinking that placing my image on the outside of the shell then painting the outline on the inside with a "paint" pen would be the fastest way to get my image on the shell.

or is it really jsut a case of spending hours and hours cutting liquid mask out?

sharpie do or did paint pens. not sure if you can still get them or get them over here etc.
posca paint pens i know are available.

i think there molotow paint pens also, think thats how you spell it.
you'll have to do some digging

You can get Tamiya paint pens. I had one with white paint in it. Used it for the letters on tyres. Sad i know . Dont know if they still do em. Google search coming up me thinks