This guy has a ton of interesting WebRTC experiments, including audio/video recording: https://github.com/muaz-khan/
Here's a demo of recording: https://www.webrtc-experiment.com/RTCMultiConnection-v1.4-Demos/RecordRTC-and-RTCMultiConnection.html
It collects the audio and video streams on the client and gives you a blob of audio and a blob of video that you can then upload/splice together.
Not exactly what you were hoping for, I think, but could probably get the job done. Hope that helps.