My experience with most conductive paint is a lack of mechanical strenght. But there are some products available for industrial use, to ease the shift from usual soldering processes to RoHS compliant production. The best electrical conductive glue I have tested so far is 2-component silver filled epoxy resin from manufacturer Panacol. It is pretty expensive but allows for repair work otherwise impossible (as for parts that must not be soldered or welded but need a reliable conductive bond). The product Elecolit 325 can be used like a common 2K-Resin curing at room temperature with enough time of 2 hours to apply the sticky mixture. The electrical properties as well as mechanical strength are impressive. See the data sheet on:
http://www.panacol.com/images/pdf/el325_gb.pdf
The company supplies samples upon request, but they will charge around USD 50 for handling and shipping a sample of 10 Grams.
So if you managed to make and align a mask to apply the conductive glue in a porous printing process, to have equal portions of glue in place of each solder ball, you could try to mount a cleaned BGA device with conductive glue. You would need to prepare a spacer also, to hold the BGA device in proper separation of the pcb, or else the glue portions might combine to a film, causing shorts. And practice with some samples before attempting to work on a living board, best on a piece of glass for visual examination of the result. This method allows only for one shot.The epoxy resin gets damned hard, when cured. It is more than likely to destroy the processor or the pcb when you try to get them apart, again. Check for electrical function after the recommended curing time, or else you might mess up the connection. Also check operation with mounted heatsink, only.