I was trying to think of other exercises that may avoid the issue, like reverse crunches like another reader mentioned, or hanging with your hands and pulling your knees upward to your stomach ... but they all seem to round the spine, as you've indicated... Damn it...
I guess it's good ol' bellydancing for me then :D
Well, as long as you are standing on your two feet, the core will always be working. Makes sure to keep the core tight in all your lifts, to effectively transfer energy to the weights. The same goes for jumping, running etc. Elite athletes talk about stiffening their core and compare it to a steel rod (no bend).
Belly dancers, funny enough, generally have alot of problem with their back, because of how much they have to move and flick their spines.
There goes my dream.
Never give up. ;)