Is it possible to use external activation on sql express without going thru sql enterprise/standard?
I would like to send a message to sql express service broker, then
You can do that, as long as the box sending the message is non-express edition.
The edition-related limitation of Service Broker is that a message must go through at least one non-express instance on its way from the sending service to the receiving service. There are no limitations on the External Activator service itself.