@girish thanks for getting back on this. Had to playing around with the apache2 configuration and was finally able to figure out a way that works. To make a long story short: ProxyPreserveHost lead to a infinite loop. As soon as I removed it, things started working.
I've put together the final app here, maybe this is of use for someone else:
Given the fact that I had to play around quite a bit, maybe we should add this to the app store as well? As a helper app, like the LAMP one.