Players using a macro to speak as a character?

Players can change the drop down menu to speak as the characters that they have access to, but I can't find a way to do it through a macro.

It's not possible.  Only GM's can use /as and /emas to speak as or emote as another character. The only thing a player can do is manually change the drop-down to the appropriate character and then type or macro text to display. The only way you can provide players a method to speak or emote as another character is through API scripts on a PRO subscription account as the GM/Creater of a given game.