Why not use photos? Person comes in, you snap a photo and send it to the referrer. References checked.

And funny you should mention it - when I got hired to Microsoft nobody asked for my ID (until the day they needed to send the stuff to the tax-man which is long after the job offer was signed). I'm pretty sure nothing chnaged since. So you see, it works well enough.

I can see the point for your portfolio, but this seems like more of a business trademark issue than personal identity issue.