Good question Bryan.
I just finished reading about a method of printing on metal different from dye sub. Direct, as you said and with aqueous inks, Breathing Color is offering it on Epson printers. It will be interesting to see how the competition heats up and shakes out. Does one cost less to print, etc? It looks like the images are better color with the Breathing Color system than the dye sub. I'm sure there are draw backs too however.