snat_add_static_mapping(...) needs a minor feature addition, namely the ability to add a specific (identity) map:
 
nat44 add static mapping external <out-interface> 53053 local <out-interface> 53053 udp
 
It’s a bit gross, but this kind of identity mapping allows DNS name-server replies to traverse the NAT.
 
The missing bit is to allow the local side of the mapping to pick up an interface address. I can imagine adding a second sw_if_index to the routine, etc. etc.