That's not really much different than adding a new keyword to the system and having a specific event set that flag to activate it.
But it is different. With the method you're describing, the new keyword only shows up at a certain point in a coversation, or perhaps at the beginning of a conversation. This presents two poential problems -- a player who knows a keyword can't input it right when they wish, and have to sort through a conversation to make it show up; whereas if it shows up beforehand then someone who doesn't know the new word will suddenly know about it because it showed up. Both are too artificial and "game-y" for my liking.
Instead, if the player had the option of typing in a word that was not displayed on the list, they could skip right to the topic directly if they wish (and if they know the keyword) while a player that didn't know it wouldn't have it spoiled for them. Simply having a list of words to click on cannot allow this kind of interaction.
Anyway, I hope this is something the folks at Larian will consider allowing in the editor if it's feasible.
