Since you are going to ask me "HolyGuacamole, which question are you answering?", I am going to pre-empt this a bit So, I am answering this question
So here I go whining again: WHY? Why can't I send application tier in the email when I can check for it in trigger conditions?
Well, you can. There is nothing a little SQL macro cannot make up for. If your application custom property name is AppTier, then use the following SQL macro in your trigger action in lieu of an out of the box variable that isn't available today.
${SQL:SELECT AppTier FROM APM_ApplicationCustomProperties WHERE ApplicationID=${ApplicationID}}
It works. Well done. Thank you HolyGuacamole.