Originally, "reset dhcp" commands worked by mangling the leases file. This is fragile and inconvenient.
DHCPD provides OMAPI, an API for managing its state, and omcmd/omshell client commands. The only problem is that it cannot limit access to it by source interface.
Apparently the only solution is to automatically insert iptables rules based on options.
I suggest this default configuration:
dhcp-server { omapi { port 7911 allowed-interface lo } }