First off, I lack alot of coding skills as I have had about a year of class in Java (I don't like it, but whatever), so I can only provide text to coding ideas. Something I just thought of a couple minutes ago, not sure how well it will run (if it will :P )

You're gonna have to follow my rough outline of how the messages will travel (feel free to explain any shortcomings of this).

A few programs that will make it run: Web Server run locally (pretty sure I saw it on the extras-devel depository), MMS program that processes the messages and turns them into plain .jpg/jpeg files to run on the web server, and alot of clever coding.

1.Original Phone: Sends MMS to destination phone number.
2.Destination Phone: Identifies it is not SMS, sends to local web server MMS program.
3.MMS Program: Takes MMS, converts to Jpg/Jpeg, sends report to messaging (not sure how yet, maybe a notification or SMS?).
4.Destination Phone: Opens web browser to local server to image folder or file manager to images or just straight to Images app (could be used instead of notification, just open Images app).

Now to do that in reverse, I have no idea :\

Anyone?

EDIT: By the way, I presume that you can steal the SMS/MMS incoming messages (hopefully they aren't just discarded).