You need to reproduce the functionality you see in SeamListener and SeamServletFilter. Note that since some of the calls from those classes pass things like HttpSession and HttpServletRequest into Seam, you would need some kind of dummy HttpXxxxXxxxx objects that wrap similar things from the Restlet API.
Thanks for the reply Gavin.
I see what you mean. I had started to change Seam code (commenting stuff out and duplicating classes such as Initialization) but I see that Seam is firmly hooked into the Servlet API.
I think you would be best placed to make the Servlet API pluggable... not to mention JSF. A job for another day!
In the mean time I'll see how far I can get in making a mock Servlet implementation that sits between the Restlet API and Seam. I hope I don't need to do the same for JSF.
I'll let you know if I have any success.