Is it possible to patch a crack in a toilet?
I'd try a marine grade epoxy (loctite makes such a product) to just seal the crack for a short time. If you still have the chunk that was broken off the marine grade epoxy should work but it's probably not going to look pretty. I've used it before and it mixes up to a rather ugly dark gray/black color and dries the same, but if you can live with the color difference it is VERY strong and waterproof. I used it to seal a leaky PVC pipe for a number of days until I had the time to fix it properly and not a drop of water came out.

EDIT: Found a link to the product here