Versions of Mono for Native Client are now available for download in the SDK. These are compiled with glibc and the libmono comes as a dynamic library. Only pepper_19 and above has Mono available.
Note that you must keep your pepper_X and naclmono_X bundles in sync (ie, don't update one without updating the other) or you will probably be unable to run the resulting built nexes. New versions of Mono that match a given SDK should be available within hours of a new SDK build being available.