Hi,
From my research, it doesn't look like its really been covered more so on the R50/R53. But if anyone has an R56 and their battery dies and key just turns in the door. There is a solution! Jack up the car and get underneath and take a portable car charger and attach the positive to the big red positive cable going to the starter and attach the negative to a good ground spot. Wait a few minutes depending on how flat your battery is and you should then have enough power to unlock the car remotely and pop the hood. And then proceed to change the battery or properly charge!

Hope this helps others.

 

Possibly an easier way depending on what you have to work with at the time:
Unscrew the 4 screws holding the license plate light cover trim.
Find the plug that goes to the hatch lock & apply 12V to it - open the hatch to get to the battery or crawl in (with some difficulty!) & open the hood.