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But, and obviously, no Boris bike rider has ever done anything that you mention, because of the registration number.
That's actually a really interesting question. One would think that there would be examples of Boris bikes being used in crimes and the cardholder that rented them getting traced, but I can't find any on a quick Google. Probably because it's trivially easy either to steal a Boris bike or deface the registration. Similar to cloned plates on cars (or the current fashion simply not to have one on the front).
I wouldn't hugely oppose some kind of anti-RLJ device being fitted to bikes, but I can't imagine how you would make it simultaneously (i) not too expensive to install and (ii) hard to remove or disable.
But, and obviously, no Boris bike rider has ever done anything that you mention, because of the registration number. No?