As NFITC1 indicates, only certain characters even get their limit bars filled, although it's theoretically feasible that an AI counter script might be able to manipulate the bar (though whether you'd even get limits when the bar is full is a completely different matter).
The usual method is to replace a char with Sephiroth. But limits are still an issue- obviously, he can't use other people's limit animations. I haven't tested to see if he can transform a la Vincent, but I believe you could use WM to have him use spells for limits. You could mod materia so that four spell slots are reserved for Sephiroth, then create your own "limits".