(Oops - I just noticed the following doesn't apply because you don't have an internal DNS server.)
BarryG recently taught many of us this trick. You're right that it goes in 'Request Routing'.
So, for example, if your internal network is 172.16.1.0/24, define:
1.16.172.in-addr.arpa → <Internal DNS Server>
Cheers - Bob
PS I don't know how to do reverse DNS with the Astaro DNS Proxy.