About the serials numbers, I can create bar codes that will correspond to specifics number (and still be directly readable), as I'm working in a packaging company and we have specific software for this. I'm not sure this will be really useful or nice on these documents, and it will be hard to do one for each, but I wanted to propose it in case you are interested.
The encrypted key is also a good idea, and it is more accessible I think since we don't own a personal bar code scanner...