Barmiracle Remote preview is available right now! All premium features are active for the duration of the game jam, provided you are signed into C3.
The bad news is that all browsers on iOS have to use apple's webview, which doesn't support webRTC yet and it's required for preview to work. You can see the current status at http://iswebrtcreadyyet.com/.
Well I think it's safe to assume that if apple's webview doesn't support webRTC yet, we shouldn't be holding our breath.
@Nepeo Construct 2 has a preview feature that uses a LAN address (which we can use to test on iOS mobile devices). Can we expect the same feature to be added to Construct 3 as a work around?