Tag provider inheritance sounds like a perfect solution. We'd have a "Standard" provider on central/master gateway with our standard UDT definitions and all other providers would inherit from this one so they always have current standard UDT definitions. EAM could sync tags from standard provider on central/master gateway to all agent gateways' "Standard" providers where all local providers on those gateways would inherit from it.
Presently, it looks like EAM send tags is the best option to sync UDTs between gateways. Unfortunately this requires a task per provider per gateway if your providers are uniquely named to allow running the same projects on multiple gateways. If EAM task "Send Tags" allowed specifying all providers (instead of a specific named provider) on target gateway(s), this would work almost as well as tag provider inheritance. One task on master gateway could sync standard UDTs to all providers on all agent gateways.
A workaround for Send Tags requirement to specify a provider would be to copy UDT configs from a Standard provider on each gateway updated via EAM Send Tags to all other providers on the gateway via script using system.tag.configure. Unfortunately this hits a snag:
Is there currently any option better than a send tags task per provider per gateway to sync UDTs between gateways?