The easiest workaround is to use a router that supports iptables, and redirect requests to the Goodle DNS servers to your DNS proxy instead. Unfortunately my router does not support this.
For thos of you who have the same issue, there is another workaround: if you have a rooted Android device, you can use iptableson your mobile. Just create two rules that redirect those requests to your DNS proxy instead. In my case, as my router already is configured to use a DNS proxy, I redirected the requests to my router:
iptables -t nat -A OUTPUT -d 8.8.8.8 -j DNAT --to-destination
iptables -t nat -A OUTPUT -d 8.8.4.4 -j DNAT --to-destination
You can enter these statements using a Terminal Emulator. Unfortunately this means you will have to execute them every time you reboot your device.
If you have AFWall+ or DroidWall, you can instead enter the above lines as a custom script, so they get executed every time your firewall applies its own rules.