I was tempted to replicate the scenario in my 2013 C250 but for the life of me, the trunk just kept popping up! Guessed this is not possible on a vehicle equipped with a keyless entry.

 

There are only 2 ways to lock the key in a car with keyless entry:
1) You are in the car and lock the doors with the door buttons or the key;
2) The key is in the car and you lock the car with the OTHER KEY.

 

I had the same situation and WAS able to get into the trunk without towing the car to MB. I disconnected the battery and waited 15-20 minutes. Reconnected the battery and then cycled locking and unlocking using the buttons on the dash (2009 CLK550). Not sure if this did the trick or if it was the fact that I was holding the Open Trunk button on the door when my friend tried the trunk latch and it opened. So, like someone suggested earlier, it doesn't cost anything to try and you might get lucky. I thought this had ZERO chance of working but it did. Hope it does for you as well.