"A professional standpoint...". Me? That's very flattering but I just come here to have fun and give some of ya' a hard time...
I would have initially had the same question as Deb. I adopted her puffer and he still freaks me out some mornings when I see him wedged in crevices havin' a "snooze".
I can't offer a solution for a "lighter material" of rock. Even if there were, I don't believe that would be the answer. Like Mitch said, opening the spaces allows for greater flow throughout the system. Personally, I'd replace the lava with live rock. More visually pleasing and you lose the risk of unwanted trace elements leaching into the system. Any time you're "entering their domain" and moving things around, you're creating a stress factor that can be avoided. There's always risks of scratching the glass as well...
If you decide to replace the rock, use caution as ammonia spikes are innevitable.