Imagine a PCB filled with a pattern of tiny copper squares, each seperated from its neighbors by a thin gap of perhaps 5 mils. Anyone who has soldered a fine pitch board knows that such a gap is easy to accidentally fill with solder when it bleeds from one pad to the next. What if we used that to our advantage; we could write our traces and pads with solder on a heated pattern of copper islands.
Given such a board with this fine copper pattern, it could be heated in an oven or on a hotpad to a temperature where solder would melt when touching the pad. Now take fine diameter solder and draw on the board where you want the pads and traces. This could easily be automated with a CNC mill and a solder feeding head.
Once the solder is layed down on the pad area, the board could be cooled, then flux paste applied where the component pads will be laid. After component placement, which again could be automated via a pick n place machine, the board would be reflowed to complete the assembly.
This method would lend itself best to single sided, surface mount designs, but perhaps a premade pattern of through hole pads could be provided and incorporated as need be into the design of each board or holes could be drilled and wires placed through to connect sides where needed.
The advantage is that the cost of each board would be minimal since the "island" pattern could be manufactured in large quantities, and each board could be different lending itself to quick prototyping and customized circuitry. Automation can easily be applied.