we figured out a way, but it is not inside the nvl dialog itself - you'll need to make some screens in your favorite graphics tool. This example produces a "full screen" video effect on the phone, then returns the player to the phone chat.
Basically, we use 2 screens:
A ('phone_frame') is an image of the MC's hand holding the phone over a background, with the phone screen transparent
B ('phone_bg') is an identical image of the MC's hand holding the phone, but the phone is opaque and blank, and the background is transparent
A displays on top, then the nvl screen from nighten's code, then B displays behind both.
The video we want to show is a ren'py Image called vid_01 that uses Movie(play="file") in its declaration.
So the text chat happens in the nvl screen, then when we want the player to receive the video, we do:
nvl_narrator "Video Stream Starting..."
show vid_01 behind phone_frame
$ renpy.pause() # in our case, player watches video loop "on" the phone until click
hide vid_01
nvl_narrator "Video Stream Ended"
# text chat carries on