Interesting question, you could type up what you need, copy and save to Evernote. I save things there for use later on. You could just paste it back when you need it. Don't know if there is an easier method though. Hope it helps in the meantime.
Just had another thought. The new shortcuts feature will allow you to create a code for what could be a few sentences. I don't know how much text it will allow you for each code though but I know it's at least a few lines of text. You say your text will be broken up by photos and such so this may be a viable option for you and you wouldn't have to leave your email and come back to it. Check it out. Settings - general - keyboards - add new shortcuts