I think thats not the best Idea, becaust 1st thing is that i don't think Opus provides 5,4 Amps on the 12V line.(Don't know the specs) And the other thing is that even if it would provide so much there would be not much left for other components like 3,5" Hard drives ore so on! Your Opus would be overloaded very fast!
I would rather buy one of those DC-DC Laptop-Power supplies (from Ebay etc) which output constantly 12V and use this for the screen.
Greets for Germany