I've set the dh_keyboard value to 0 and rebooted the device.
Now I'm definitely seeing all the stuff I would expect to see in the console.
Okay, as a reference, here is all I did:
Based on:
Dahua Enable Telnet
First, on NVR change admin password to "admin"
http://<myip_address>/cgi-bin/configManager.cgi?action=setConfig&Telnet.Enable=true
Authenticate as: admin/admin
Telnet to my ip address.
Use login: admin
Password: admin
Use this command to turn off dh_keyboard:
busybox bootenv dh_keyboard 0
Reboot.
Now, the RS232 console is showing the full trace logs of the NVR. I'm hoping it reboots and I can get some logs!!!