If anyone wants to try out the approach outlined above, then here are some links to the images I used. Note that this most likely will only work on DCS-930L revision A! If you have the B1 version then it probably will not work. Use all of it at your own risk, this is highly experimental at this stage 
- Initial image to flash through the emergency web interface: https://www.tributary.se/openwrt/openwrt-ramips-rt305x-dcs-930-squashfs-factory.bin
Note that this image is over 5 years old (Barrier Breaker, r39854) and guaranteed to be full of security holes! It is only for getting OpenWrt onto the camera initially, don't run this image on any type of network that you don't fully trust. I have successfully flashed this one on two cameras of A1 and A3 HW revision respectively. To flash use a paperclip to hold in reset when powering on the camera, and then go to http://192.168.0.20/ where you find the emergency web interface. I had some problems using it with a regular browser, instead I used curl as per another post:
curl -F firmware=@/tmp/openwrt-ramips-rt305x-dcs-930-squashfs-factory.bin http://192.168.0.20/
If flashing works the camera should reboot after a minute (red and blue leds flashing). Once the front led starts blinking green the kernel has been successfully loaded and the camera is booting. When the light is steady green the camera should be reachable. Default network config will put it at 192.168.1.1. As mentioned ssh login does not work, but telnet does. Once logged in either scp in the next image, or set a password to allow scp from the outside.
- I made the image I created (based on OpenWrt 19.07.6) available here: https://www.tributary.se/openwrt/openwrt-19.07.6-ramips-rt305x-dcs-930-squashfs-sysupgrade.bin, and the full config is here: https://www.tributary.se/openwrt/config-DCS-930L_A1.
Before going to the next stage it can be good to check in /proc/mtd that all partitions show up. If rootfs and rootfs_data are listed it should probably be ok to proceed.I upgraded without keeping settings, otherwise you most likely will inherit some ancient settings from the initial firmware:
sysupgrade -n -v /tmp/openwrt/openwrt-19.07.6-ramips-rt305x-dcs-930-squashfs-sysupgrade.bin
After a minute or so the device should reboot, and if the upgrade was successful boot up with leds going green again. If you see the front led going red and the back one blue in 10 or so second intervals something went wrong and it cannot boot. You can revert back to either the D-Link firmware or the initial one above through the emergency web interface in that case.
If the reboot works then you should have a functioning camera, if enabling mjpg-streamer. You can try it manually as well with the following:
mjpg_streamer -i "/usr/lib/input_uvc.so -n -r 640x480 -d /dev/video0" -o "/usr/lib/output_http.so -p 8080 -w /www/webcam
The network is hardcoded to 192.168.1.1, changing this to DHCP or adding a default route + DNS is needed to get NTP to work, unless you have an NTP server on the local network.