You can absolutely do that - I've succesfully used a Dahua DVR as an "IP encoder" to record to a Vigil NVR - just give the Vigil the same rtsp:// URI you'd used for a Dahua camera, but add a modifier for the channel desired (eg. "?channel=3"). Don't see why you couldn't do similar with a second smaller Dahua... or any other brand that supports similar custom URIs. Might even work with ONVIF.
The other question is, would the primary DVR handle the multiple output streams any better than the cameras? It would be the same total load on the network either way, just a lot more traffic for the DVR's port to handle, sending out 6*3 streams.
Of course, if the execs don't "need" full HD video, you COULD have your view-only DVRs pull substreams from the cameras directly, at far less bandwidth.
You could also look into multicasting from the cameras and/or primary DVR, but that's getting beyond my experience now.
Which model DVR/NVR are you using?
I know that there are Hikvision and Dahua DVR's that output RTSP on each channel. Not sure if the NVR's do.
Do you have a portscanner?
One solution can be using the DR-4132P or DR6232P(S) NVR from IDIS, you can hook-up a DD-1116. The DD-1116 will then act a a embedded RTOS Client. Just connet a mouse via USB and a display on the HDMI and you will have a workstation like enviroment.
The DD-1116 will pull the stream information from the NVR, but the fisical stream will come from the camera side. Options like event pop-up are also embedded in the DD-1116.
Sincere thanks for all the feedback! I feel much better having a few options as a backup in case pulling the 3 streams from the cameras prove problematic.
Another option that has worked well for us is the Geovision GV-IP Decoder Box. It can decode either an ONVIF or rtsp stream directly from the camera. Many of the other decoders seemed to work only with a particluar camera manufacturer. It does have a max limitation of four 1280x720 streams though.
I understand that it's exactly the opposite of what you were asked for, but a pc with your vms software client and a little Windows scripting could probably do this. I've got a bunch of wall mounted displays that we've done this with where we want something to run with no user interaction. Auto reboot on power failure. Auto launch client on boot. Auto login client. Not much to it. VESA mount mini computer to the back of the display.