Yeah - there is a full uboot replacement project (linked from that repo) - so you can basically build openwrt for it.
That project I linked too is more designed at end users who just want to replace the userspace with somethat that isn't tied to xiaomi's cloud services. So is the best starting point.
I didn't even bother looking at the firmware that comes with it; Serial uart is exposed so I wouldn't worry too much about the users harping on about not being able to install versions from newer factory firmwares ; serial allows you to push whatever you like.