As the developer of RocketAMF http://github.com/warhammerkid/rocket-amf, I don't know of any AMF libraries that can act as clients out of the box. However, if you're interested in it, it shouldn't be that difficult to reverse the server code in RocketAMF to work as a client. You would just write a serializer for RocketAMF::Request that uses the standard message calling style (#<RocketAMF::Request:0x10167b658 @headers=[], @messages=[#<RocketAMF::Message:0x10167ae88 @response_uri="/1", @data=["session stirng", 42.0], @target_uri="App.helloWorld">], @amf_version=3>
). Then you would write a deserializer for RocketAMF::Response.
I'll try to put together a new RocketAMF build in the next couple days that can communicate with FMS, but it's not a guarantee.