If I have a player dead to rights trolling on replay but namedrop him, will Riot take no action?

Not on the boards for sure.

There's one way to find out. Make a support ticket with the information (the way you should post this information), and let support take it from there.

If you "namedrop" him on the boards, Riot will ask you to remove the post and/or even punish you for making it. You have to use other channels which people posting here have already talked about. Sadly, there's more protection for intentional feeders than for people who lose matches because a teammate decided to ruin the game for their whole team.

So the short answer is - don't post it, use a support ticket like hethalean says.