I think I found a solution, although I am not 100% sure it fully complies with the TOS of Adsense. It is explained in DFP's help section:
Define "Inventory 1" and place an order from an ad-network. As a fallback in the ad-network do not use Adsense directly but use another Inventory of DFP (called "passback inventory"). This way you give back the control to the DFP server which then decides which ad to serve.
Enable Adsense for the passback inventory. Nowhere in the process you are using the "Adsense Code".
The only problem left is how Adsense will recognize the page it is displayed on. When using Adsense from a DFP tag that is placed in the HTML it should be able to recognize the content of the page. But if the Adsense is called from a DFP tag that is a fallback from an ad-network there is a chance that the parent page is not recognized and the content unknown... (DFP's GPT tags work with iframes behind the scene),