From what I can tell, looking at some of the code (https://github.com/jupyterlab/jupyterlab/blob/ac3ab8cbed744cbbaacf4871a7a9e152ef6c9086/packages/logconsole-extension/src/nboutput.ts#L32-L71) it will display any iopub messages with priority info. So if you do any normal outputs from Jupyter, (printing, IPython.core.display.display, etc) these will show up in the output widget?
Although @jasongrout would have more ideas probably…