The 400 Series was originally going to permit custom RGB sensors or tracking modules on the D4 board, connected via the 50-pin 'Tracking Module' connector on the board. Intel decided eventually not to support customizations on this connector though.
I believe the starting process for arranging an NDA is to open a support ticket at support.intel.com
With most hardware that has a storage space for firmware, you could likely write your own custom firmware. The D4 has 16 mb of serial flash for firmware storage. The problem would be knowing where to even begin without open-source firmware code as a reference.
Prior to March 2018, the D4's tracking module connector and MIPI details were documented in the 400 Series data sheet. Although custom sensors are not supported in the current official firmware, it will give you a lot of details about how this aspect of the D4 is designed.
In case there was future interest in this connector, like your inquiry today, earlier in the year I created a PDF extract of the tracking module section of the data sheet to save it for future reference.
The MIPI details can be found on page 90 onwards of the January 2018 edition of the data sheet.
Up until the point where tracking module connector support was discontinued, it was stated that once a custom sensor was connected (probably via via a custom-built flexible cable so the pins on the other end of the cable match up to the custom sensor's pin-out) then the camera should be re-calibrated. The custom sensor should have functioned after that.
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The RealSense firmware for the D4 processor is Intel IP and details are not shared with anyone, even under NDA. If you want to use the D4 processor with your own custom cameras then you will have to write your own firmware.