Ask away. Do you really think that many people are going to take the time to witness a screening in a private room for a stranger? Most are going to think that it will take too long, it will put them in an awkward position potentially seeing someone exposed, or that it would put themselves at risk.

Most people have no issue with airport screening as it stands; finding someone who would be willing to be a witness would be a challenge, I suspect.

I do not recommend that any traveller assume that they can easily find a witness in the allotted time.

Again, I think that it is poor advice. You may be fine with it, but it puts people at more potential risk than a public screening at the checkpoint.