This doesn't work with iMessage, because the carrier sees iMessage no differently from any other kind of data download. However, a friend of mine tried doing so

This doesn't work with iMessage, because the carrier sees iMessage no differently from any other kind of data download. However, a friend of mine tried doing something similar using the <number>@txt.att.net (or equivalent for your carrier) email-to-SMS trick and wrote an Android app that parsed the messages. They promptly received a firmly worded email from AT&T and were forced to abandon the project.