So the first part is - done. I can now toggle x10 devices on and off from Clojure. This just used off-the-shelf Java libraries, and most of the hour or so it took was spent dealing with the Java environment and documentation
The hard part is going to be the web interface. That will be a Java servlet, probably using Jetty as the server. However, it needs a framework - and there aren't any I consider useable already available for clojure.