Thanks for mentioning the float valve idea. That had come up before in another thread.
A toilet vavle may have the required components, however I am not sure if they can be scavenged to be retrofited for a drip system. There are several float valves on the market that have threaded inlets and outlets and will lend quite well to a drip system application. My only consern is that float valves require a certain line pressure to unseat the gasket. This won't be a problem as long as the pressure regulator is after the float valve.
By the way, the valves with threaded ports are available through any plumbing supplier or swimming pool supplier (frequently used on commercial pools).