Configuring the dnsmasq instance started by connection sharing in NetworkManager
Dnsmasq doesn't "share connections". The role of the NetworkManager-controlled dnsmasq is to route DNS traffic.
You can't configure the NetworkManager-controlled dnsmasq process in Precise. In Quantal you are able to configure it via files in /etc/NetworkManager/dnsmasq.d/. See dnsmasq(8).
Another possibility is to use the standalone dnsmasq server which is fully configurable even in Precise. To run standalone dnsmasq alongside NM-dnsmasq in Precise you need to give standalone dnsmasq the "except-interface=lo" option. This is not necessary in Quantal.