Agreed on netgroups. The use case of 100s of clients on an export is ideal for netgroups. That way, you only have to apply a single netgroup for a centrally managed set of hosts.
Therefore, automation wouldn't even be needed.
LDAP, NIS and local files can be used for netgroups.
See TR-4379 for best practices, as well as the product documentaion if you want to use local netgroups.