He's talking about using the 138 degree C solder (Tin Bismuth eutectic).
That still doesn't leave you much margin, temp wise, though. (Oh, and if you used non-ROHS parts on it, they'll form a tin/lead/bismuth eutectic with melting point of 98C, which is why they don't use bismuth solder much).
As you mentioned, solder paste is supposed to form a ball if you melt a little bit of it. This is governed by surface tension, and is in fact critical to the functioning of solder paste. If it didn't have that behavior, the solder wouldn't shrink away from the solder mask, and you'd solder on an IC and all the pins would be shorted together by a big blob of mess. Old solder paste will behave that way, due to oxide on the metal particles. In your case, though, that surface tension becomes a problem, because it will make the solder try to ball up instead of staying in the grooves where you want it. And what will the flux do to the plastic?
With indium, you can make alloys that melt at a low enough temperature to take a cast of your fingerprint without burning yourself.... But this doesn't solve your problem!
The most interesting "circuit printing" thing I saw would print conductive plastic, as well as normal plastic, and was meant for making the case/frame (so your case would be your wiring harness). Not sure what the conductivity of that plastic was though - most conductive plastic sucks; I stopped looking at the kickstarter page when i saw the price, which was before I found the conductivity.