I'm pretty certain you will not be able to pop a copy of Spiderman 3 into your Audi satnav slot and have it play on your MMI screen, you're going to need additional hardware.
I'm no expert but the first thing to be aware of here is that DVD has different uses. Satnav will use a DVD-ROM data format and hence the Audi sat nav system will be designed to read that data, plan routes and render maps etc on the screen.
Video DVDs i.e. films use the same basic DVD discs but use it to store video/audio data in MPEG format*. These MPEG files need decoders to be able to take the data and render it into images/audio not to mention some way of handling the encryption used for DRM, menu structures etc. Generally this is achieved by dedicated hardware (i.e. microchips in your DVD player rather than embedded software).
*Anyone who has ever tried ripping a video DVD (naughty!) will know it's actually a little more complex than this with the video/audio stream being held in VOB containers etc but it'll suffice for now
My guess is that you'll need an external device to do this i.e. some form of portable DVD player to decode the video from the DVD disk and pass the audio and video streams to the MMI display.
As Doubledoom says, you then need some way of hooking up the portable DVD player into the MMI display. So, what you're really trying to achieve is to get your MMI to act as a mini LCD panel rather than as a DVD player.
Again, I'll stress I'm no expert in this field and there may be some inaccuracies in my comments but I think the basic principles are correct.
