So restrict what people can enter in certain fields. You're missing the point. I have to agree.
Any restriction is likely to cause problems at some point.
In my original post, I had not considered the possibility of machine generated code. But now having thought about it, it all makes sense. My original proposition though, didn't call for restriciting, rather picking an combination of characters unlikely to be chosen by the user. Allowing for any possibility and every eventuality is one of the major strengths of Unix and why it has lasted so long.