Disclaimer: this is client side only, obviously.
Navigate to the folder as per below and find GAMEPLAY_AUDIO.txt:
Find the hero you want to update and change the text you want.Code:*:\Path to Hearthstone\Hearthstone\Strings\Language\GAMEPLAY_AUDIO.txt
For example:
Whenever I click the Greetings emote with Nemsy, the text displayed will say 'Waddup!'. (The sound file played will remain the same)Code:# HERO_07a is Nemsy Necrofizzle (Warlock) VO_HERO_18_Female_Gnome_Greetings_01 Waddup!