J, you should be fine with just a few mm. When I say 1/4" or 3/16", I'm thinking of things like ash, mahogany or limba. Plus, it can't be really thin because I need to set the magnets into it. Were I using something like ebony, or rosewood, I'd probably go with less than 1/8" thickness. I've no experience with zebrawood, but you can glean from that what you will.
Also, what's wrong with just leaning towards the safe side, making the zebrawood cavity cover thin so you don't gouge the body, then laminating some other wood onto the back of the zebrawood cavity cover? It's not like anyone will ever see it.
peace,
russ