The only way I would attempt it would be to soak the rock in the cleanest rodi water possible (000 tds) and keep changing the water every few days over a few weeks. I'd also use a copper test kit to monitor the copper levels that leach out of the rock. Finally, I think i'd let the rock dry outside for a couple weeks before attempting using it again - however I'd porbably NOT EVER use it in a reef again - maybe a fowlr tank.
Sadly the low cost of dry base rock probably would be a safer option longterm than trying to save some rock that was exposed to copper.
How much rock are we talking about that was exposed to copper?