If the RMTs position was sufficiently strong, alongside industrial action they’d by now also have attempted to present at least some of their evidence in court, reasonably confident of success in some areas. However relying almost entirely on public disruption through strikes suggest there’s almost no comparison whatsoever with the likes of Uber.
So how would prove that the former guard was a member of the RMT or actually took part in any strike action? You would be on a very slippery slope. It's very akin the likes of Uber. You would be discriminating against someone based on their lawful actions. That is pretty much saying that anyone who goes on strike is unemployable in future.