The problem with your multiple device solution is that it puts burden on the recipient, who's highly likely to have any idea what NOFWD is. The fingerprinting solution you outline next is highly flakey as well, as it would require a critical mass of users to gather the necessary data, and even then the kind of algorithms you'd need are orders of magnitude more complex than the ones Facebook etc. use, as those catch instances of users logging in from foreign countries (or perhaps states), and would not catch cases where the 3rd party is in the same city, a very likely case.

A typical scenario I imagine is that the recipient will lose the message, and end up emailing or calling back for the same information, which will annoy all parties (or possibly worse, they'll lose the information and ignore it forever), as well as require the information be transmitted more times than usual (something someone security conscious probably won't like).

I hate to be so critical of this service, but I just can't see this being useful. I wouldn't call this hitting a nail with a sledgehammer, it's more like hitting a nail with a six foot ceramic feather.