Can I download games-on-demand from foreign market places?

How are you using a fake IP address? Microsoft may be using geolocation (detecting the real IP and finding the location) which filters out your content prior to location or anything to detec the location of your IP address.

Possibly, they use your Xbox serial numbers, or whatever to identify the Xbox and where it may have been purchased (if you purchased your Xbox in Germany, then it's probably certain why).

Im not sure of full games but I downloaded the Saw 2 demo (not available in Australia) by creating a U.S. hotmail account, then using that to make an Xbox Live account. Once you sign in, you have access to whatever country your hotmail account is linked to. The demo then works fine.