has anyone had any success mounting these directly to the ceiling plasterboard or is the only safe way to screw mounting hooks/bolts into the joists directly?

I've had luck mounting directly to the sheetrock, but my 4" panels only weigh about 12 pounds. How much do yours weigh that you're concerned about needing joists?

As I have said before here, I made a frame from 1x2, routed a rabit to accept 1/4" lauan, and made a 'tray' to hold the 2x4 foot panels. I wrapped the panels in muslin, and glued them into the trays. The 1x2 rails helped to hide the wrap, and gave me a solid surface to handle physical attachment to the ceiling. It also allowed me to put a spacer block to get the panels a little bit raised off of the ceiling, to slightly improve performance. (I blocked up only one side of some of the panels to make them sit at an angle, just for looks. With a 1x2 rail, you can use screws, toggle bolts, or whatever you need to hold the panels to the ceiling.

My ceiling is white, and I painted the trays to match the ceiling before I glued the panels to the trays.

Bill