I spent a little more time today trying to build and run XML-RPC.NET using Mono. It was frustrating. There seem to be some major shortcomings in the implementation of HttpWebRequest, at least on the Windows 0.17 build. In particular, setting the Proxy property is ignored (as is it setting it globally using GlobalProxySelection), closing the request stream before calling GetResponse causes Mono to crash, and Content-Length handling appears to be broken when handling the HTTP response.
Once I get the imminent XML-RPC.NET release out I may look into this, the first step being to install Mono on Linux and check whether the same problems occur.
UPDATE: In fact I already had RedHat 7.2 running in a VMWARE image and it only took a couple of minutes to install the necessary RPMs for Mono. I coded a test program with the XML-RPC request hard-coded in a string and cofirmed that the same problems occur (although with different behavior when Close is called on the request stream).