I see what you mean Ian; it does seem like one would need a little back pressure to get a steady delivery.
Jason, wow, that's great! Just this week the idea of syringe pumps hit me, but it's great to know that others have already been thinking about this stuff.
I've been mulling over a design in my head... imagine that the syringe (or pump) is seated on the counter, next to the kettle, with a length of plastic tubing leading to a hypodermic needle which is held in position over the pot by a lab stand. The stand could clamp a cork, through which you push the needle.
Regarding an 18-gauge needle; this is the size that seemed like a good fit to me, but I haven't used it in practice. I'd be very curious to know why you might be looking for something smaller; was it not ideal?
I was able to find some 18g needles in 4" length, which seems like a good length to work with. (here)