Is it possible to homebrew edible silkscreen ink?
This doesn’t really help with the ink question, but this Wafer Paper would be very useful. Instead of printing directly onto the Fondont, you can print/write on this paper and then transfer it to the cake.
You can ink-jet-print on rice paper with edible inks, but you must either be able to clean your printer very well beforehand or have a printer dedicated to printing only edibles. Regular ink-jet ink is poisonous. Edible frosting sheets The sheet is not actually frosting; the site says that it will dissolve into the frosting of your cake.