Great thinking, but I doubt it'll work that way. Depending on the legal systems of different countries, you could have no criminal record to speak of yet be well versed in all manner of illegal and dangerous activities. Not to mention you don't necessarily need a criminal background to be a danger to someone. If you look at the background of a lot of lone wolf terrorists and the like, they may have had disciplinary problems as kids but rarely ran afoul of law enforcement. I guarantee if some idiot college student Tweeted "Goin' to London and the Buckingham Palace - can't wait to blow that place UP!!!" and MI5 caught wind of it, he wouldn't be enjoying tea and crumpets in merry old England. He'd receive a similar treatment and, on their decision, be allowed entry or refused.