Don't even think about overwintering it in z6, even in a cold frame it's going to be too cold too long: this is a z8 tropical, and I'm guessing you've got it in a pot, which means it's less hardy than in the ground.

Are you sure you can't find a place indoors? What kind of pets do you have? We have a dachshund, so he couldn't reach our potted lantana up on the radiator if he tried, but I haven't heard of cats nibbling the leaves -- it's the berries that contain the most toxins, the leaves are actually made into a tea in the Caribbean, where it's native.

If you bring it in for the winter you would need to prune it back severely anyway; lantana goes semi-dormant in the lower light conditions of an indoor winter, so there wouldn't be much foliage on it anyway.

Hope this helps,

Jim