Welcome to the forum Tim.
Many dashcams have a AV out option ( this is only low resolution not 720 or 1080p ) some even have a mini or micro HDMI out, and these can be HD / FHD but i am not quite sure they allow for video to a screen while recording.
Having a screen with video going, even if it is video from the road in front of you, well it might be illegal if the driver can see the screen.
I personally see dashcams as a stealth technology so the less other people see of them the better, i can easy imagine in a situation the other part can go crazy if he know you have it all on video.
This could easy become violent if the other part are that kind of low life scum,,,,, and sadly there seem to be more and more of those, almost to a degree you could claim it is trending.