The only problem that could come up from this is, if two people get put on the same team which both "locked" their favorite champion, and it happens to be the same champion, who gets their choice?
You can't logically allow something that doesn't happen normally (allow both players on the same team to have the same champion), so logically one person would have wasted a reroll (aka safeguard) for no benefit.
It would require a significant amount of work for them to track which champion was safeguarded last by a player to avoid placing two players with the same safeguarded champion on the same team, so much so that I doubt it would even be on their radar for consideration for a very, very long time.
Not trying to burst bubbles, but just presenting a logical counter to the question. =/