[Reelin]

Sure, you can report any arbitrary key as positive. I can even do it right here on HN; my positive key that I just made up is "0d d8 cb 25 8a 88 aa df 6a 33 17 5f 59 ad fd bf"!

... now what? Someone has to aggregate that key (along with all the other flagged ones) somewhere, and then end users have to voluntarily choose to download the keys from that source and check for themselves if they came into contact with it. So you would have to get someone to accept your self reported positive key, and then convince a bunch of end users to trust that (apparently untrustworthy) data source.

I expect that most databases will require some sort of authentication from a healthcare provider or known laboratory before they will accept a key from an end user.