Confirmed to work with at least Kamikize 7.09 (2012 Oct 04)
The process I followed:
1. Upload this DD-WRT mini through the regular WNDR3300 web UI.
2. WNDR3300 will reboot into DD-WRT and be set to default 192.168.1.1.
3. Browse to http://192.168.1.1, set credentials, then browse to the administration tab and upload an OpenWrt firmware. I used this 7.09 WRT150N firmware:
4. WNDR3300 will reboot into default OpenWrt.
5. Like the WRT150N, you will need to "ipkg install kmod-brcm-wl-mimo" and "wifi detect > /etc/config/wireless" to get basic WiFi working. To the best of my knowledge this supports only 2.4GHz – 5GHz is not working but that could be my own limited understanding.
May work with other builds but this was my successful starting point.