Can Silkscreen Go Directly Onto Copper PCB Layer? (No SolderMask)
I've done this in the past, but it was unintentional and before the silkscreen actually got printed on the copper, the manufacterer removed it before it got to production

The best people to talk to about this would be the manufacturer, usually when you do print on copper they'll send you an email or just remove it. They'll be able to tell you if it will work or not and since every manufacuterer used different silkscreen formulations and ways to print (some are like a dot matrix printer) they would be the one to ask.