This modality was tested a few times, the last one at the round table at the end of the Interdisciplinary Symposium of Complex Systems 2014
https://sites.google.com/site/complexsystems2014/
In this round table, one of the experts was blocked in Oslo by a strike, and participated via Skype.
We experienced some audio problems due to the lack of time for organization. In principle, the remote expert would need to see the slide projected, the other experts and the public. In order to accomplish it, one can either use a frame grabber for capturing the output of the pc projecting the slides, or use the same computer to project the slide and to participate to the hangout, plus other two PCs (or a software video mixer) for changing the views. A digital audio mixer is also handy for input/output audio from the pc connected to the remote expert.
Another example of this modality (with just one expert and no slides) was during the “blue nigh of research” 2010
https://sites.google.com/site/nottebludellaricerca/