Reports in champion selection would be actually completely useless. You can just report them after the game.
The purpose is for games where a player is trolling etc. and forces someone to leave the match...you can't report them at the end if the game is never played, yet they have still broken the rules and will do it again. The only option is to submit a support ticket with a screenshot, or play the game with them just so you can report. Those are shitty alternatives.
What would have a beneifit, is a dodge for free button.
I'm not talking about free dodges. I'm talking about leaving champ select and incurring your normal dodge penalties, nothing more. Currently if you want to dodge you have to exit the entire client and relog.
A function like this is only useful if it has an effect instantly. But if it has to be instantly an check by Riot if trolling actually happened is excluded.
Reporting is not to get a benefit for yourself instantly. Reports are to help protect others in the future. Having reporting in champ select they could view the logs, but as it stands now there is no match record of a game that was never played, and therefore no chat log.