I've also been looking into this situation. My frustration with the RSE system is, my kids are too young to work the RSE unit and it's nearly impossible to deal with from the front seat. I'm tired of pulling over and climbing into the back seat, every time I need to change a DVD for my kids. Shame on you BMW, for not considering the fact that young children might want to watch a DVD while their Parents sit in the front seats!
After doing a ton of research, I've learned the RSE is completely isolated from the main A/V system, with one exception; you can listen to audio from the RSE through the main system. Furthermore, there's no video output from the RSE, other than the LVDS cable that connects the RSE DVD player to the RSE screen. There is no way possible to "tap" into this connector and connect it to the main system, and vice versa. The aftermarket isn't interested in developing the appropriate interface because the market for it isn't big enough.
Here's how I'm dealing with it in my 2011 X5: I am mounting an aftermarket DVD player in the glovebox, where the OEM DVD changer would be located. One video output from the DVD player will be connected to the unused video input pins in the connector on the back of the RSE screen. The second video output, and the L&R audio outputs, from the DVD player, will be connected to an aftermarket A/V input interface (
NavLinkz Shop | c.LOGiC lite BMW Professional CIC | buy online) which connects to the main screen. This will even allow me to control the DVD player with my iDrive. I'll also connect the audio out to the audio input for the RSE headphone control unit.
I'll certainly report back when I have the installation completed.