Those are some neat projects you’ve linked.
They are using a laser printer, not an inkjet printer. I’m not sure why the silver paste is sticking better to the glue than the paper, but I’m guessing it’s in the chemistry/physical properties of the laser ink. Inkjet ink probably won’t work.
Are you trying to make circuits inside 3D printed parts, like the first one, or are you trying to make your own 2D, 1 layer PCBs like the second one? If the latter, using the MPCNC as a PCB mill is probably easier and better. I’ve also seen someone use an MPCNC (I think it was an MPCNC) to make an etching mask. They coated the whole PCB with something black, then used the CNC to scrape off the ink/paint where they wanted isolation, then etched the PCB and cleaned it off.
For making 3D printed parts with traced installed, aren’t there conductive filaments? I think the key would be to print that stuff (if it’s hard) and make the MPCNC dual extrusion. I don’t know how well just silver paste would work if you extruded it into a cavity using a CNC machine. Rework would be pretty tough!
Interesting ideas. Some very clever stuff here.