Is there a special reason you are interested in the DNCs?
DNCs are implemented using a vector format (VPF), what we can call as well a specific SENC or internal format for displaying vector
charts.
VPF is not implemented in
OpenCPN.
More information about VPF:
Vector Product Format
What would be the benefit for
OpenCPN implementing VPF?
From the information here:
Differences Between NOAA ENCs® and DNCs®
it seems that the NOAA ENCs in S-57 format, which are supported completely by OpenCPN, are covering the same content with at least the same information.
ENCs are even more often updated than the DNCs.
From the DNC web site I understand that for the non-official users only charts produced and with
copyright from NGA or other US agencies are available.
(remember the NGA
project here some time ago...)
bcn