Official webrtc on ios is sometime to come. however because Cordova allows native code to be accessed by JS. All hope is not lost. There is a group out there doing a WebRTC layer for PhoneGap. I'm not sure if they are doing a 1:1 function calling though. If they are then PhoneGap is your access to WebRTC on IOS in the mean time.
However depending on the nature of your online game. You can still use WebSocket for fault and corrective realtime(ie non precision realtime), you can use comet based ajax, or you can even use nodejs websocket or comet commet communication. The only downfall to this is that you need to do more effort on programming your own server side work.
Does MP work on IOS yes, does not the MP WebRTC plugin work on IOS not at this time ....