I have a shelf mantle piece, 54" long and 4" top to bottom. It is hollow except for a support board across the middle. It is surprisingly heavy, about 30 lbs(?) Any suggestions on installing it onto the brick above my fireplace?

You could use bracing brackets to hold it up after you use masonry screws to bolt them into the brick. I would use 3-4 brackets placed evenly apart-and get the ones that slide over the screws-just like your towel bar holder. You might want to screw the brackets into the mantle and use it as one piece. They have some very decorative brackets made just for your dilema. Only problem is that you need to have screws long enough to drive into the brick-and make sure they are a snug fit, and don't forget to use washers between the screw head and the brick. Would hate to have you put expensive items on the mantle only to have the mantle fall off. The other option is to screw directly through the supporting bracket into the brick. Only problem is on every bracket you'll be able to see 2 screw heads-unless you fill them. Then again there might be other ways to do it-might want to ask someone who does this type of work for a living. Good luck!!