I ended up trying to do the refilling thing on a food-grade cartridge at work earlier this year. We had an "edible image" to print out for a cake order and the one color had run out. It took up to a week to order in a new one, and we didn't want to disappoint the customer.
I ended up getting a small kid medicine syringe (blunt end-no needle) from the pharmacy and loading it up with the comparable color from our bottled colors and peeling the label back and taking out the tiny rubber stopper and refilling it a few drops at a time till I had enough to print.
My hands were bright hot pink for the better part of a week.
I've always wondered, how DO those "photo cakes" work? Do you print the image with edible ink on an edible transfer, and then just stick it on the cake, or is it done by airbrushing?
Ah ha! I can help you there! I set a cake printer for a client of mine.
The ink is food coloring, as you have guessed; the paper is very very thin, edible rice paper, different from the one we use to make Chinese lanterns and kites cause it is made with just rice and water, not glues or toxic chemicals. So the printer prints it, then he places it over the frosting, and then tops it with gloss jelly (that shiny cornstarch thing they put on fruit pies), which moistens the paper making it edible.
Then you can all eat a piece of the poor child whose picture is on the cake. I am creeped out, but hey, different strokes...