In the rest_ws plugin for restful web services, client pooling is built into the plugin. It would be nice to have this same option in the HeadlessClient plugin. I have written some code in a module to simulate pooling (simple round-robin approach) with headless clients, but real pooling can only be done in the java side of the plugin.
This is useful for some customers I have that deal a lot with third-party API's where lots of requests are sent off server-side via some API, and then when the responses are received they update the database to record the response. However, some requests can take a while to process, but you don't want to have to spawn a new headless client instance for each individual request.