(Though you cannot call just any method asynchronously; the interface's IDL needs specific annotations and the underlying object needs to implement ICallFactory.)
Just add async_uuid to your IDL locally and rerun MIDL. The server doesn't know or care that it's being called asynchronously. It's a pure client thing. If you're calling an IFoo in a different apartment, you can do the call asynchronously by changing your client configuration without the IFoo implementation having to be aware that anything is going on.
You can also use IRpcChannelBuffer3 to make the call asynchronously no matter what the IDL says. You'll have to write the proxy plumbing yourself, but it'll work, and you don't even need to rerun MIDL.
Just add async_uuid to your IDL locally and rerun MIDL. The server doesn't know or care that it's being called asynchronously. It's a pure client thing. If you're calling an IFoo in a different apartment, you can do the call asynchronously by changing your client configuration without the IFoo implementation having to be aware that anything is going on.
You can also use IRpcChannelBuffer3 to make the call asynchronously no matter what the IDL says. You'll have to write the proxy plumbing yourself, but it'll work, and you don't even need to rerun MIDL.