I somewhat agree with knowing if the enemy is flashed. Could possibly become somewhat abused. I still think it'd be interesting to experiment with for a week or so and get some collective thoughts afterwards.
As for knowing if your team mate is flashed, I don't think adding an alert into the chat would be useful as most players don't look at chat while they're in the zone. Also in bigger rooms where many players can be hit by flash this would cause spam in the chat. Which is why you suggested the additional chat filter in options. So that's why I suggested some kind of HUD symbol instead. I think having this symbol for team mates would be a nice addition to help enforce team play.
If anything do trials for both ways for both aspects (just team vs enemy) (chat vs symbol/alert) and get everyone's opinions on it after a week. I appreciate your perspective on my suggestion.