www.textnovel.com already have this feature. Their FAQ describes their system as follows:

"• The subject line must have the following format: Story Code:chapter number
If the story code is 12 and the chapter number is 2, then the subject line should read 12:2. If you want to add a new chapter, then make sure to use the next number in order (so if you already have 10 chapters, to add a new chapter you would use 11 as the chapter number in the subject line)."

But perhaps a more friendly system could be created