Bit of an awkward one in some respects . Some may see a wonderful period fireplace needlessly damaged for the sake of fashion while others see just a pile of ugly bricks.
Personally I'm never fond go painting brickwork , if it's ugly it will just be ugly in a different colour , and if it's decent it'd be an absolute pain to remove should someone want to in the future. The same thing applies to plaster albeit to a lesser degree.
Another option and one I've done myself , although not to a fireplace , is to fix grounds or batterns to the brickwork taking care to fix only to the joints not the bricks themselves and then to board that with plasterboard , mdf , or even that cement based board ( aquaboard? ) then finish as desired , plaster , paint , timber or tiles.