is it possible to build a WebThings like a camera/video camera but to support an mjpeg stream?
Yes… theoretically.
The only video streaming we’ve tested thus far is through the ONVIF add-on. To make things work well in a web browser, we transcode all video streams to DASH and HLS, since MJPEG is not natively supported by all browsers.
The key is to use a VideoProperty, with one or more links, such as this.